Yes back then 77 PE250 was road registerable without indicators, Things only started getting tigher around 79 when Australia requested suzuki Japan start putting blinkers on PE's to meet ADR rules NSW was first. Other manufactuers did the same but not everyone had blinkers at the same time. Some makes only got em in the early 80's and left it as late as possible. All the new ADR/compliance plate issues took a few years to fully come into effect. Now fast forward to 2010 things are a little different. I believe in NSW you need to have blinkers on anything road registered no matter what but they may have exemptions for some old 1920's bikes that are not capable of running blinkers and would just look plain stupid?. In SA, you dont need to have blinkers on the 77 250 or any other PE or bike up to 1993 for that matter. Even if the bike was older than 93 and came out with blinkers, you are allowed to remove them. I do not know anything about Victoria or other states though. So even though you may have been able to register the 250 with no blinkers back then, your new state rules may now require them or may not. You will just have to contact Victorian transport department. Actually if you got to their website you should be able to find all the rego requirements for your state in PDF doc to download. i know all this stuff is on Transport SA site but i have never gone looking for it on the Vic site.
With the number plate i also have a feeling it may have only been an optional item for racing use. ive certianly seen many magazine tests with out them and i doubt the magazines would have automatically taken them off for photo shoots etc, they would have wanted to show the bike as it was with everything on it.