I have the same guitar. Bought it in 1987/88. It was used and came with the original Hoyer case with orange plush lining. It also has the original Hoyer humbuckers. As a young and fairly ignorant guitar player, I did the mistake of replacing the humbuckers with newer pickups around 1992. Those are still on the guitar, but I regret not having the originals on it. From what I can see, your guitar and mine are more or less identical. The volume knobs on mine cracked and fell off a few years ago.
Welcome to LTG! It's always great finding an owner of one, as they are pretty rare to find. I have seen a few and all are somehow different... Are you in the US or in Europe?
The case is a real Hoyer case but Hoyer never made a fitting case for that model, so they simply sold some that were close enough.
Great looking piece of wood used for the body of your 50603!
Both GAD's and yours are definitelly from the same vintage, probably just a few serial numbers appart. They are from the first run of those guitars and I'd date them around 1975.
EDIT: Looking at your guitar, your guitar is slightly younger than GAD's one. GAD's one has no pickguard yet, they came a bit later as far as I found out, still might be from the same year or maybe 1976 or so.
Oh, and yes, the Hoyer knobs had a great tendency of cracking, I was also looking to find some black knobs, but no chance...And I'm in Bavaria... (that's not my guitar below, I have no Hoyer currently, just an example picture)
And one aditional hint: Be extremely careful with the two switches, the plastic levers of them also break very easily and are totally impossible to replace. But that shape of them is like a signature feature of those Hoyer models...
One more question regarding the pickups.
Do you have any recollection how they looked like? I'd assume like this:
Not like those, this was the 1976/1977 split design, where a rotating switch was used to split the 3 bass from the 3 trebble pole pieces and recombining them between the bridge and neck pickup to get more possible tones out.
Ralf