[Lagerfeldt]

This is from Wikipedia

6581 R1 - Prototype, only appeared on the CES machines, has a datecode of 4981 to 0482 or so. Has the full 12 bit filter cutoff range. An unknown number were produced, probably between 50 and 100 chips. All are ceramic packages.
6581 R2 - Will say "6581" only on the package. Filter cutoff range was reduced to 11 bits and the MSB bit disconnected/forced permanently on, but is still on the die. Made from 1182 until at least 5082. First 10 weeks or so of chips have ceramic packages (these usually appear on engineering prototypes but a few are on sold machines), the rest have plastic packages.
6581 R3 - Will say "6581" only, "6581 R3" or "6581 CBM" on the package. Had a minor change to the protection/buffering of the input pins. Made from before 2083 until 0486 or so.
6581 R4 - Will say "6581 R4" on the package. Silicon grade changed to HMOS-II "HC-30" grade, though the manufacturing process for the chip remained NMOS. Produced from 4985 until around 2586.
6581 R4 AR - Will say "6581 R4 AR" on the package. Minor adjustment to the silicon grade, no die change from R4. Produced from around 1986 (week 30) until at least the year 1990.
6582 - Will say "6582" on the package. Typically produced around the years 1989 and 1990.
6582 A - Will say "6582 A" on the package. Typically produced around the year 1992.
8580 R5 - Will say "8580R5" on the package. Produced from the years 1986 to 1992.

BTW, my sample cd includes a full range of C64 drums by Jeroen Tel (Maniacs of Noise), sampled from his very own C64. I think he did the best drum sounds on the '64.