Its a 1985 model.
As well as the previously mentioned tank shape/radiator shrouds, I'm 99.9% sure that the 84s had drum front brakes.
Further to Leith's comments about the build date, be VERY wary of trusting the compliance plate dates on Bert Flood era KTMs. Unlike the importers of the Japansese brands of the day, Bert 'manipulated' the scheme to his (and the buying public's) advantage.
Short version is that the 250s and 300s were usually all complianced as 250s, while 350s and 500s were all complianced as 400s.
Also, if he had old compliance plates, they'd get used up on the new model bikes.
My old 88 125MX was complianced as an 88 125GS by BFI.
My old 89 250GS was complianced as an 88 or 87 250GS by BFI.
A few other bikes I've looked at are similar.