I found a little bit more information in one of the many books on gun values. This book (name escapes me for the moment) includes serial number ranges, so I take it as hopefully just a wee bit more accurate than the often contradictory information in other books.
1919 was the original .25 model, there was also a 1926 and a 1926-31, then in 1936-37 Beretta produced the 318 for a 'limited run'. The 418 started production in 1937 and went on until 1961. The later of these guns had serial numbers ending in a, b, or c. (the gun I bought is a 418, not truly a 1919.)
Based on information taken from various books, and a collection of pictures of particular examples, I would rough out the timeline of changes for these guns something like this:
The original 1919 had the early-style tapered grip frame. In 1936 the grip was changed to a slightly flared and rounded shape for the 318. In 1937 for the 418 a loaded champer indicator was added. Around 1950 the grip safety was changed from a rounded protrusion to a crescent shape following the frame; around 1953 the slide serrations were angled. These middle-period 418s were the ones marketed in the US as the 'Bantam'. By the late 1950s further changes were made including flatter grip panels which curve back to follow the back of the frame just under the slide, which variation was marketed in the US as the 'Panther', which is printed on the grips (several sources list the 318 as the 'Panther', but the examples I've seen referenced were made in the late '50s).
Comments/corrections?
[edited with additional information.]