Yes, that's the plate . Wouldn't think someone would just put plate on. Is the date on hsv plate the same or close to date on other plates. Car should have badges on car. Can't make out from photo does cold air intake have hsv on it. Hole exhaust should be different to Holden one , mufflers will have hsv stamped on them . Should probably have 185 badges on fuel rail covers. You could go into your local holden dealers spare parts with chassis and engine numbers if you get on well with them they can look up, I think it'll tell them all options and specs car came out with.
The other one to try, might be quicker and easier is ring hsv with your chassis and engine numbers . I did this before handing over money for my maloo, you'd think they'd have records for hsv enhanced cars as well.
If they do should be able to get a certificate of authenticity, think it costs about 120