There are four incarnations of this lens in the marketplace right now, rechipping issues aside, and in order to straighten them out I phoned Sigma's Customer Service because their lens chart isn't up to date. The model number is actually six digits, with the last three digits identifying the mount not shown here.
All four are 70-300mm F4.0-5.6 plus the following descriptions:
model # 506 APO Macro Super II, has a red ring on the barrel
model # 508 APO DG Macro, a newer lens that replaces # 506, also has a red ring
These two are the better lenses and very close in price. The newer lens is almost an ounce heavier, not sure why.
model # 507 DL Macro Super II
model # 509 DG Macro, a newer lens that replaces # 507
These two are the lesser lenses; less contrast, color definition, and sharpness according to Sigma and should be priced about 30% less, too.
I've seen eBay sellers trying to pass off over-priced 509s as an upgrade to the 506 by saying they're newer. It's "newer", but not "better", so be careful out there if you're bargain hunting.
Since the APO Macro Super II is disappearing from stock here and there, I thought it might be useful for people to know that the APO DG Macro is the next generation of the same lens.