Glad to see you have the LST,but what you pasted is not as important as the codes are,they are what you use to look up parts with.
I read over the thread with the fork issue and to answer one question...no,there is pretty much nothing in the top cover/shift rail housing that's interchangable between the T-34,5 & 6 and the T-495/496. The forks are totally different being the main difference.
The cause of the fork breaking can be a number of things things though. Usually it's something in the cover like a bent shift rail and even the detent balls and springs can be at the root of problems,if any of these aren't up to snuff and shifting becomes harder some will begin to force it instead of fixing it while it's a simple matter,hence something breaks that shouldn't have. If I read it right the fork was the rear one which is 1st & rev.,theres a "lockout" block for 1 & R that can cause problems if the plunger/spring are fubar which as I said make somebody force things. Out of the 3 forks we sold fewer 1/R than the other 2,the primary reason there was drivers using the shift lever as a hand rest and putting presure against the syncro's,that would wear on both. A one point IH revised things and added nylon inserts to those forks in effort to lessen the wear factor. It helped but didn't aliviate the problem totally.
The parts book you have is not right for the year truck you have,the MT-127 covers 1971 to 1973. You need the MT-132,it has the 495/6 trannies.
It took me about 10 minutes to look up and Google the nut,even after all these years I still know the illustration figure plus a lot of the pn's.
There are however many internal parts shared by all the above mention trannies and some are still available thru the web.
In case you're unaware a member (Irish Mike) has a site with a number of parts and service manuals.
Here's the MT-132
https://app.box.com/shared/otkihx6yoa/1/26421826It's an older revision but is better than nothing. Unfortunately there isn't one for the T-495/6 but there is one for the T-434,5 & 6 which is the short lived designation for the T-34 et al. As the internals are virtually the same as what you have it should be useful.
https://app.box.com/shared/otkihx6yoa/1/55882590These manuals can be downloaded in whole or part, I do it in sections for the ones I have,doing an entire manual at once takes hours.
FYI, Theres a little box with up/down arrows to change how things are viewed,I prefer numeric/alphabetical so I opt for Name when I use the site,it just makes it easier/faster.
Overall I wholly concur with CB as I'm a believer in "do it once,do it right". If you can find a reman unit it may be the best route,that way you have a warranty. Finding parts all from one source is iffy at best.Buying a used one is even more iffy because it's true condition may be no better than what you have.
Btw,
The 495/6 was further "upgraded" ca.1980 and redesignated T-698/699 respectively,used in the S-series well into the 80's. The direct in 5th models are much more prevalent than the OD models (ie; T-34,496 & 699) and are way easier to find. Being you have a 2spd rear I doubt you'd really notice a big difference anyway if you went with a direct model.
Off the top of my head I can think of 2 possible sources for reman. units and possibly even some parts. Six States Distributors and Weller Truck Parts. Crane Carrier (CCC) is another that may still do these trannies.
http://www.sixstates.com/products/products.jsphttp://www.wellertruck.com/Both have locations in Boise.
12 yrs.exp. in IH dealer parts dept.
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