The Mark VIs were always marked as such, while the SBA has no model markings. Early Mark VIs were marked to the Northeast of their octave buttons, while later Mark VIs have a stamp on the very front of the fitting ring where the bow & bell are connected. Other than the Mark VI stamping, there are two easily discernible differences between a Mark VI & SBA. The first of these is that the Mark VI left pinky cluster (pic below) has linkage between the low Bb and low Db keys (the link acts as a stop for the low Db key's travel). The second visible difference is the neck octave arm. The Mark VI has an arm that is stamped from flat stock, with a large blue 'S"  (well, the blue DOES wear off after a while) at the fulcrum (area where the arm attaches to the neck), while the SBA has an octave arm made up from round stock (in the 'cigar cutter' style).

The cause of much of the confusion over differentiating & identifying amongst the SBA/Mark VI/ Mark VII models is two fold: 1) multiple assembly locations; and 2) Selmer's failure to maintain model integrity within the serial number sequences associated with these various models. Though Selmer parts were made in France, Mark VIs were assembled from these parts in multiple locations. Notable assembly locations were Paris, Elkhart, Canada & Great Britain. This practice minimized import duties on Selmer products in countries where there were significant sales volumes. It is common for import duties on parts to be less than those on finished goods because parts will presumably provide in-country jobs for assembly and/or maintenance workers. Therefore, it is also common practice -- spanning myriad industries - to export parts for assembly into finished goods in-country. The hands of governments & industry wash one another, it would seem.

It is this multiple assembly location strategy that is the main source of the variations we see from one Mark VI saxophone to another. The reason is simple: Talent, facilities & equipment varied from one assembly shop to the next. Because assembly centers such as Elkhart had rich resources inherited from the American vintage saxophone industry -- like a population of master instrument engravers -- we see the horns assembled there with more intricate engraving patterns. Because of this huge saxophone-oriented talent pool available in Elkhart the instruments assembled there are very highly regarded as to quality. The Elkhart horns also tend to have serial numbered necks (no doubt due to local management philosophy), whereas neck numbering from the other assembly areas is spotty (and was eventually discontinued altogether late in the Mark VI model span).

Fortunately, a preponderance of Mark VIs were assembled at Elkhart. That's because the USA instrument market was bursting with demand in the post WWII era. Local preferences may also play a part in why Mark VIs assembled for different markets vary in their characteristics. For instance, silver plated saxophones were more popular in Europe, so we see more plated horns assembled in Paris...and because the engraving talent was not so widely available in Paris, these silver plated Mark VIs destined for the European market often have little or no engraving. While the Mark VIs aren't marked as to where they were assembled, there are these broad trends in their cosmetic features that lend to matching them to an assembly location. The other tricky thing with Selmer models is their failure to maintain a homogenous design throughout their saxophone line over time. So as not to appear to be singling Selmer out in this trait, the American saxophone makers had their transitional periods, too. The following information may be of some help in sorting anomaly from standard Selmer practice. Selmer spilled some models over from the SBA run into the Marl VI run. I know this is true of the bass saxes made up until at least 1958 because I own an SBA bass of 73xxx serial number that is clearly NOT a Mark VI (by its features). This instrument also has an asterisk preceding the serial number rather than the 'M' prefix that started with the Mark VI run. I've not encountered any other 'spill overs' from SBA models into the Mark VI numbers, but there may be some. I would suspect sopranos & possibly low A baris could exist in this SBA/VI interface, though I've not actually seen them. While the SBA/Mark VI indiscretion appears to be of a transitional nature - meaning there was eventually a full array of Mark VI models - the changeover to Mark VII from Mark VI was never completed. Whether that was because the Mark VII was not as successful as the Mark VI figures into Selmer's thinking is unknown - at least here in the Oklahoma green country. The practical application is that there were never any Mark VII bass, bari, soprano or sopranino saxophones, though there are Mark VIs of these types bearing serial numbers in the Mark VII range.

I sort of glanced off your question about the 'M' prefix that started with Mark VI models, eh? The real answer is 'I don't know for certain what the 'M' means'. Since it first appeared as a prefix to the Mark VI serial numbers - but only when the instruments were of the Mark VI design - there is the obvious presumption/assumption that the 'M' might indeed be a model indicator. Assumptions are dangerous, though. For instance, Conn adopted an 'M' prefix for all its saxophone serial numbers in 1925. It would be easy to associate that 'M' with the introduction of the 'real' Chu Berry models that also occurred at that time. Fact is: Conn used one serial number sequence for all its woodwinds, so the boom in instrument sales of the early 1920s created a need to be able to differentiate between instruments (clarinet, flute, bassoon, saxophone) as a matter of post-sale customer support. In this case the Conn 'M' prefix was a code that stood for 'saxophone' (I'll leave it to your imagination why an 'M' and not an 'S'). Since my last parts order from Selmer took in excess of six months for them to to fill, I really doubt their motives for prefixing the Mark VI serial numbers with an 'M' were based on customer service concerns. What I do know with certainty is that when you are armed with the above knowledge, that 'M' serial number prefix has little relevance when faced with the task of distinguishing a Mark VI from the SBA or Mark VII Selmer models.