Small zero with a line under is 1960. I just bought a 1960 Don, that's the only reason I know. lolFor some reason (not an intellectual one), I favor older pipes. By older, I mean 60s or earlier. The earlier the better. Not sure how to verify this is specifically a 1960 as described by the seller.
And, as long as it is not too beat up... I actually prefer to restore a pipe. It adds some intrinsic value to it for me. Yeah, I am kind of weird.
I would be remiss if I didn't point out both Maxim (Pipes2Smoke) and Dave Neeb (Mkelaw-Pipes) have some good prices on some Dunhills and both are site sponsors as well as great guys to deal with.