If this was type NM cable I agree that the individual conductor type is unknown but all SER cable that I have seen uses type XHHW conductors and the conductor type is marked on the jacket. The individual conductors are also frequently marked with their type.
From the 2005 NECH:
According to the 2004 UL Electrical Construction Materials Directory, category TXKT (service cable) and category TYLZ (service-entrance cable rated 600 volts) are listed in sizes 14 AWG and larger for copper and 12 AWG and larger for aluminum or copper-clad aluminum. Type SE cable contains Types RHW, RHW–2, XHHW, XHHW–2, THWN, or THWN–2 conductors. Type USE cable contains conductors with insulation equivalent to RHW or XHHW. Type USE-2 contains insulation equivalent to RHW-2 or XHHW-2 and is rated 90°C wet or dry.
Curt
