Let me be clear; Music Ground uses their ownership of the name to produce fake vintage pieces which would have been manufactured years before that company existed. They then hit various forums on the internet talking up new "previously undiscovered" vintage Hiwatt (and other desirable brands') models that just happen to have features that should be anachronisms on amps of that vintage.
I'm going to stick my neck out here & say there's elements of accuracy on both sides here. The trannies AND IEC
are correct for a 78 Canadian Hiwatt (I couldn't find a 77 to refer to); BUT the rear chassis isn't. IF it was a Canadian amp, yes they were hard-wired to 115, but Hiwatt still used their standard chassis and rivetted at blanking plate where the mains selector should be.
In Hiwatt circles Mark Huss is generally regarded as the guru on their history & I've linked to this page, for reference.
http://hiwatt.org/pix.php?p=hiwattguts02IF the spec plate on the one in the OP has the dropped parenthises then I'm afraid it's 100% that it's been "adapted" in some way by MG & that any info written on it should be taken with a pinch of salt. I don't see that on the OP pics but what I do see, call me pedantic, is that the "Hiwatt Amplifier" at the top of the plate is much different.
My guess? It's had a new chassis made in the dim & distant, though not necessarily by Hiwatt or MG.