Your thoughts on the trigger seemed to be accurate. I researched the Internet further and found a old post about the Diana 50 (see below). It compares triggers and trigger guards to access the date of manufacture. Below is a picture of my trigger and guard, one small adjusting screw and no hole in the guard. These were the same as the prewar triggers and were probably used up in the earliest years of production.
“The model 50 entered production in May of 1952, and I believe any gun with the small adjuster screws must have been made within a couple years of that date. I would make an "educated guess" your rifle is from 1952 or 1953.” From posting below.
“I believe that these small adjusting screws were actually pre-WW2 parts, that Diana used up in their early post-war production. To illustrate the point further, this shot shows three model 27's. The gun at top is pre-WW2, note the small adjuster screws. The middle rifle shows a later variation of the post-war trigger, with a ribbed face. The bottom gun is a 1980's model 27 with stamped steel trigger.” From posting below
Here is the entire discussion which I quoted:
https://www.tapatalk.com/groups/Amer...0-b-t2965.html