I have a lot of magazine tests from Two Wheels. ADB, T&T etc on the 77 and they all have rego plates on them (front and rear usually) and i don't recall any mention saying that you could not. Usually they soul say things like. 'Great bike but can not be registered which lets it down' or something similar.
VIN numbers first came in 81 but bit on everything. My 82 PE250 does not have one.
Loke Pokey says, it may have had a sticker on head stem or maybe it just had the frame number and although compliance plates came in 75, the bike still may have been 'complianced' and 'registerable' The actual plates on PE's only came in 78 or at very latest 79 and part way through 79 blinkers were fitted too.