That would be the SM420. Check over on the Links page, there's a couple of sites with info on that tranny. And if you don't have a shop manual, check out the Online Chevy Manual site. I think that there is a '49 manual on there, if not, any of the later manuals would have info on the trans as well, as it was used up to '66.
I can relate to the syncro problem you're having. A guy I ran around with in my youth had a '49 1 ton 4 speed. It did the same thing.
