Robert,
Semmer's has 1894(1905) as 131264-133042,
1900(1905) as 350208-354620.
I glanced around at some 1900's online and found a 352K and a 353K serial numbered gun, both with improved locks. Also found a 350K gun with standard locks... which would seem to prove a mid-1905 changeover as well as Researcher's and my gun.
Of course, I did find an oddity.
Gun #132037 is a CEO Pigeon Trap and has the improved lock.
Perhaps the higher graded guns started getting the new frames first while the lower grades used up the standard.
It will be interesting to hear where Ross' 132k gun falls if he checks back in.