966b loader

just purchased a 1966 cat 966 b loader sn 57h310 and Im trying to find a sevice manual and a parts book for it cat does not have this one on there computer system yet they started with the 966c, and did this 966b come out with a tubo on it or was that a add on by some one else.This one has just got a straight pipe on it for exhaust and would like to put a muffler on it,if its okay to do it.Thanks

Uses a D333aT engine. Muffler not offered on other applications of that engine although extensions are an attachment option.
With only 434 966B 57H models built it's unlikely you will find manuals. I'd go for the 75A versions.
Check ebay for parts manuals.
https://www.ebay.com/itm/CAT-Caterpillar-966-SERVICE-SHOP-REPAIR-MANUAL-WHEEL-LOADER-GUIDE-BOOK-sn-75A/392300694272?hash=item5b56f18b00:g:H1wAAOSwMY5c5KJ8
or
https://www.ebay.com/itm/Caterpillar-966-Traxcavator-Service-Manual-SN-75A1-Up/143207398089?hash=item2157d37ac9:g:kZ4AAOSwLk9cq7s9

For some reason? they combine the later 75A with the 57H parts book.
https://www.ebay.com/itm/Caterpillar-966-Track-Loader-Parts-Book-75A1468-56H1-To-Last-Built-Sept-1978/263759706119?hash=item3d694db807:g:6CgAAOSwjzJbGUHH