Over the years I've seen my share of Parker Hammer guns, my observation for what its worth:
The top lever guns seem to be pretty standardized and were mostly made in 12 and 10 gauge, the frames were made in different sizes depending on gauge and desired weight. Most are on the heavier side, I believe the locks for 12 and 10 gauge gun are pretty much the same, never seen and 8 gauge so couldn't say. I've owned 1 16 gauge but it was several years ago (1970s) it was an 0 frame size but don't remember if the locks were smaller, 16s are rare and more so for 20s. I don't know if anyone has made a survey of the hammer guns on how many made by grade, but looking through the serialization book it's apparent that most were 0 grades. The higher grades seem to be based on the same components as the lower grades just more file work, polishing and embellishment. I believe that the Parkers were the the best quality of the mass produced American made hammer guns.
Get yourself a copy of the two-volume set of The Parker Story to learn most of everything known about Parker Bros. shotguns until its publication in the late 1990's including the tables at the end of each of the Grades section listing the number of each configuration of each grade and the quantities of each produces.
It is a very informative set of volumes on the Parker Gun and all things Parker. Well worth the investment for those who are students of the Parker Gun.
look on ebay or contact Double Gun Journal to order a set.