Well, that turned out to be a great suggestion. The museum's curator sent a detailed response within less than a day.

Apparently the O.P.P.'s original service revolver since the 1920s was the Colt Police Positive. The Mk. IV Webleys, acquired initially from various sources, were issued as supplementary handguns to new constables starting in 1946, when the force expanded considerably. There were several batches of guns shipped, although there is a gap in the records in the first half of the 1950s (which I may have narrowed by one year with the copies of my invoice I attached). By 1956, the number of Webleys in inventory was increasing even though the force was growing, which leads them to the conclusion that the Webleys were already being pulled off active service again. In the late 1950s, the Colt PP Special was then introduced.

So the total number of O.P.P. stamped Webleys may have never reached four digits, maybe the reason why they barely show up in collector forums and discussions.