I was a bit suspicions about as well. There looks like there has been a logo of some sort removed at the bottom of the blue vee. It looks the same shape as the old Sport M logo and didn't they make supporter jerseys in the early 90's? It seems a very odd choice to fake that jersey.
That's a good detail pickup and I agree it is a Sport M copy so buyer beware.
Having a passion for older league jerseys from the 70's/80's I did a quick list of all the teams original jersey manufacturers and when they first changed, for those looking for genuines rather than copies. I do stand to be corrected so give or take a season perhaps:
1978: Canterbury, Manly and Souths - from Westmont to Classic
1979: Easts and Newtown - Westmont to Classic
1982: Illawarra and Canberra - Classic
1985: Balmain, Parramatta and Norths - Westmont to CCC
1986: Penrith - Westmont to Peerless
1988: Gold Coast and Brisbane - Peerless
1988: Newcastle - Good Fellows
1989: Wests - Westmont to Sport M
1994: St George - Westmont to Peerless
1997: Cronulla - Westmont to Nike
Essentially I'm pretty sure all jerseys from the 70's were made by Westmont up until 1977 (they didn't necessarily carry the Westmont logo) when the then NSWRL brought in licensing and a logo change for all clubs for the 1978 season. From there clubs changed sporadically to different manufacturers over time.
I do know Westmont have been no more for many years, and also Classic do copies of their old jerseys (and some Peerless ones) in their Heritage range, so I gather they picked up the rights from those two manufacturers.
For example: Brisbane (1988 Powers), Cronulla (1988 Aussie Duct), St George (1979 Penfolds), Parramatta (1982 Hardies), Wests (1979 Victa), Norths (1991 Norths Leagues) and Newcastle (1988 Henny Penny) are all sold by Classic Sportswear even though they never did the jerseys at the time. As stated they must have bought the rights to those jerseys to be able to release them as such.