The Miyata 721A serial number is SK54271.
It has the follwoing components:
Derailleurs - Shimano Exage 500ex (with on OE date code)
Brakes - Exage 500ex (also with OE code)
Shifters - Shimano Light Action (SLA500)
Rear hub - Sealed mech. EXAGE FH-HG50
Front hub - HB-RM50 (with NJ date code)
Seat - Turbo
Seat post - Sakae CR
Chainring - Biopace (this bike is similar to the 718 in that it doesn't have a triple chainring!)
Crank - Shimano FC-A500 (with OB date code)
7 speed Cassette.

Cheers,