The 3-ton probably has the big filter. The 51001 is for the smaller filter like on a 235.
Seems like there was a discussion about these GMC filters recently on oldGMCtrucks.com. You need to sign up over there anyway if you have an old GMC. Sign up and do a search.
Come to think of it, it may be on the main site (not the discussion forums)at old GMCs. Check that out too.
