Distributor IAD6003-2F is Case p/n VTA2758 used on VA & 400B tractors and maybe others. Distributor IAD6003-2J is Case p/n G11051 used on 300 & 300B tractors and maybe others.
These distributors are identical except the letter designates the the governor spring part nr. The governor spring dictates the advance curve. I don't know about the letter B, but assume it has the same function as the afore mentioned IAD6003-2 distributors. Personally I would go ahead and use it on any VA, you probably will not know any difference. I presently am using the VA distributor 2F on both my 300 & the 311B B/C I was able to buy them new on ebay. I have used the 2J on on my VAC13.