People keep tossing around the term "nelson" as if its a standard. it is not. I own over 50 different types of pumps, mostly nelson based and let me tell you nothing is standard. power tubes come in all different lengths, widths and depths. valve seats are different threads across the board and have different sized holes for the power tubes with different types of seats.
hammers are all shaped different with different sized sears. bolts vary in length(not just breach and bore), and sear edge.
springs vary in length and width for both main and valve not just tension.
pump arm length differ even in the same gun line depending on the year.
alot of early nelsons were hand assembled with what ever parts were available at the time. milling was done by eye, lay a bunch of Bushmaster pump handles side by side and no 2 are the same.
so if you want to know if a part from another gun will fit you need to be specific in which gun you want, and even then it mite not be right.
I sold a 12 gram changer for a Razorback to a member, he asked if it would fit on a Trracer(old style). I checked buy putting it in my Trracer and gassing it up, worked fine. He got it and his valve guts wouldn't thread in, they were different.