Drag Radials vs Slicks

I went to check out the Hoosier Slicks I had mentioned in my earlier post. They are a D06 material where I misquoted saying D05. They have 6/32 of rubber left on them and are in very good shape. I gave him $80 for the pair. The only thing I am not sure of is the age as they didnt have a marking like a standard tire to tell the manufacturing date. Is there a code that would tell me that? Two things lead me to go this direction, 1st was the price was to good to pass up and the second is that I already have the BFG TA's for the street. I just need to buy 2 rims now.
Rod