Very curious! I think I have a good answer for what you have there, and it's really interesting. So, my guess is you have a Martin Master Model without the additional pearls. My guess as to why is that perhaps they had a Master Model body that never got sold or something, or perhaps it was a special order. I realize it is slightly too late to be considered as part of the traditional serial number range for the Master Model, but oh well. So here's my evidence.
Martin Handcraft Master Model (Typewriter): American Makes & Models / Martin / Handcraft Master - "Typewriter" / Eb Alto 101356 |
Martin Handcraft "Transitional" Model: American Makes & Models / Martin / Handcraft / Eb Alto 108502 |
Martin Handcraft Imperial: Martin Handcraft Imperial Alto Saxophone (1933)
So if you look at the first example, the typewriter model is, other than the extra pearls and finish of course, identical to your horn. I believe you already made a note of this. Also, the neck design is the same, having the added neck brace, which none of the original Handcraft models had. Also, we know it's not a neck from a different horn, because Martin put the serial number on their necks as well, and it matches the body. Lastly the engraving is exactly the same. While it is true that some of the original Handcraft horns had this design as well, I haven't been able to find a single example of a typewriter model that didn't have that shield emblem and font.
If you look at the second and third examples size by side, they are quite interesting. The second has no discernible markings as an Imperial, and like Handcraft models has a split bell design. What is also interesting is on one of the bands it has "M203" etched on. Is that meaningful? Who knows! The Imperials, as I believe you noted, moved both bell keys to the left side of the instrument. However, the key design is virtually identical, from the auxiliary F# to the sculpted high E. These are features that the typewriter model and your instrument lack. There are likely other similarities and differences between the second and third examples, but I think they suggest that the second one is the transitional horn, and perhaps even a late-stage prototype.
So, what I think this all suggests is that you in fact have a Martin Master Model, just without the "Typewriter" part of the equation. Also of note is that when compared to the original Handcraft models it has no G# trill, nor does it have the fork Eb tonehole. The original Handcrafts had these features. The Master Models did not. So yeah, seems like you have quite an interesting horn! If only it could talk...