I think the UK almost always used torx screws for 500s... My very first 500 (1.2/red power led) had them, 1.3 500s had them, the 500 plus had them. I used to work at a couple of shops as a saturday job and occasionally repair 500s, and I seem to recall it was always torx on the 500s... Could be wrong tho :-D
As above, get a set as they're dirt cheap from places like Maplins (not sure which country you're in, but there'll be some cheapo tool place near you ;-) )
