Lost reverse on a 1276

[Norms1276 2]

I changed the fluid in my 1276 hydro-gear transaxel, and lost reverse.  Unit seems to want to work in forward, but in reverse, it will not even back over a 2x4.  Linkage/cam in OK, belts not slipping, fluid level is OK.  We have owned this thing since it was new in June of 1967 so I just have a thing about getting rid of it, so would like to fix it.   I got some help from daveoman 1966 in his answer to nerd on his 1266.  Difference is the model number, just how much variation is there in these units?  Mine is 90-2001, nerd, was xx-xx46, saw another 46 for sale. Are they fully interchangeable?  Parts listings would indicate they are.   Any thoughts?