I ain't ei8ht, but.......
The magazine safety depressor, aka, Tansley device was added in 1926, according to Brunner.
He further states that the exact date of the first pistol so equipped is rather nebulous,~05-27-1926, but the serial on the 1903 was #468097 and the 1908 was #92894.
Several other modifications were made to accommodate the new setup.
Now I feel like I poached your post.
I won't lose any sleep over it if you won't.