For whatever it is worth not all IAD 6003 distributors used the same springs. That is the only difference, all other parts are the same.
Two eggs amples:
IAD 6003 2J for 300-300B 148 cid engines used spring p/n IGB-252S.
IAD 6003 2F for VA 124 cid & 400B 148 cid engines used spring p/n IGC118BS.
Obviously there is going to be some difference in the advance curve. I have used the distributors interchangeable on my 300's and VAC. I don't notice any performance difference but my tractors are lightly loaded put putters.