At first I thought maybe an inner support for the rear deck hanger mounting bolt. They sorta line up in the parts list you put up. Couldnt see how it would work and neither of mine had anything other than the hole in the plate for the transaxle to mount to and a spacer/collar.
Looked on the computer for pics of any ranchers ive pulled apart. Found pics of an autodrive, its got real similar to the older ones.
Then I remembered that I had a PDF for a Rancher on my PC.
I just had a look, ive got the PDF for the 1766 & 1744. whats the one you put up, the pics were slightly different in mine.
Part 20, transaxle support bracket.
On the back corners of the transaxle there are two threaded holes. Thats the bottom of the brackets.
The tops go in two holes in the rear of the frame at bolt 26.
They stop the transaxle from rocking.
Cheers.
