I can't say with certainty that your cranks are not anodized, but they sure look like it to me. I have a pair of anodized 600 cranks on a 1985 Bianchi that had the same (flat/yellowing/plastic) appearance. I used Easy-Off and a Scotch-Brite pad to remove the coating and polished using a cheap Harbor Freight polishing wheel set up on my grinder. They turned out real nice. Had I taken the time to sand before polishing, I probably could have removed some of the larger scratches, but it wasn't worth the time to me. I did polish the wheels, seat post, stem, chainring, RD, and shifters. None of the other parts were anodized. I spent about 3 hours polishing the cranks.

To be sure, try using and aluminum polish on them and check results. You should know pretty quick if they're anodized. Best of luck!

Before:



Bare aluminum:



After: