I am pretty sure its a 'B'. Should have a serial plate on the lower portion of the chassis. Because the tin, rims and overall body are in good to excellent condition, I would say around $700-$800 minimum worth. Get 'er running and put some new paint and decals on and she'll be worth a few thousand.
I am still not sure if that is a 'B' or not. I keep looking at that steering column and shaking my head. I don't think I have ever seen a 'B' with a completely horizontal shaft....I have only seen tilted ones on 'B's. Looking for a picture of what the serial plate would look like on the 'B'. I would assume is is similar to what is on my 'H'.
AH HA!.....It's actually a 'MT'....the tricycle version of the 'M'. A pic of what it should look like is here: http://www.retiredtractors.com/Popper/M.html
Little side note from my history. Two of my great uncles built a loader and frame to attach to the 'MT'. They used it to stack round bales when round bales become popular and few tractors had loaders yet. My father used to watch them use it and said it was terrifying to see this wobbly tricycle tractor try to handle a 600 lb. bale in the air. Always figured it would flip sideways or end over end.
The 'M' was made to replace the 'H', and was built very similar to the 'B'. One keynote difference is the steering column. It's also one of the first JD's to have verticle cylinders. Built between 1949 and 1952. 14 hp on the drawbar and 18 hp on the belt. Here is a page with more history and pics of the M and MC (crawler) as well.
I spent a lot of hours on the Int. "M" too. Dang steering wheel would break your arm when hitting a fire ant mound. We had a Super "M". Cranked on gas and ran on "Tractor Fuel". Boy, it was powerful in it's day!