An excellent post, Louis, particularly the detailed photographs. This is just the sort of thing that is of value to the MS community, present and future.

What a fortunate acquisition for both you and the rifle itself. It's great to see a fine and formerly neglected MS in the hands of one who will properly care for and cherish it.

Here are some comments:

First, I really like the 'old style' Prince of Wales grip. I don't know when those were phased out, but it seems to be a feature found only on very early MS. Per prior posts from Kuduae, the production of commercial MS rifles and stutzen of the M1903 and M1905 models began in 1905, along with mass production of the 'Greek Contract' Y1903.

I am envious of your sight hood. It was offered on the pre WW2 MS, but I have seen very few examples 'in the wild' on auction listings or on forums such as this.
Here is one listed in a 1935 Steyr catalog reprint:

There are reproductions available of a later type for the 'postwar' MS: Gunbroker MS Hood

Your folding peep sight (also an original option) has been modified. I can only presume the ball added to the top of it was for very long shots with a stutzen? The 'special folding peep sight' was of the same type used by Westley Richards, who also used the same takedown system as the pre WW2 MS:

Westley Richards peep sight

Peep sight on MS

From 1939 Stoeger - "Special folding peep sight... $12.00", also "sight protector (hood)... $1.00".

The +.05 and -.05 are to match chamber depth of receiver to barrel and are original. Your other MS should be similarly marked. I have no idea. however, what the 'Y' and 'X' suffixes are. You are correct regarding the 'circle T', and I don't recall if I've ever found an explanation of the 'circle V' that seems to be typical on the MS.

I have a wild guess regarding the lack of proofs, but it is just that... an unsubstantiated wild guess... here goes;
With 1905 being the year that Y1903 (military) production commenced, along with the M1903 and the M1905 with its new proprietary cartridge, could this be a prototype that was 'shopped' to ordnance departments in effort to gain a military contract for the 9X56 Mannlicher Schoenauer?
It seems that I have seen photos of M1900 sales samples that were not proofed.