I believe there was also a 7701, offered very briefly before the 7721. The 4 and 5 row heads fit that.
9501s are out there, too, and there may have been corn options for them.
About the only place you saw these critters was in the Dakotas and Canada... and corn in ND was not as popular then as it is now.
65, 96, 106, 6601 would take A LOT of work to make. Not quite like building a 60 crawler, but it wouldn't be a weekend job.