A bit of a mystery, still a very good find.
The reason I asked about it being for a 750 as I've had a chrome set of exhaust pipes that looked exactly like the black chrome set you've found. These turned out being from a 750 GS Suzuki, they had the smaller dia header pipes, I think 39mm OD from memory. Also the heat deflectors screwed to the collectors, near where the riders feet would be, are not found on the 1100 Kat pipes.
I've also spent quite a few weeks earlier last year rebuilding a very rusted out orig Kat 1100 exhaust system, so I sort of know the system rather too closely actually!
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Have a look on Mick Hone's website and enter those numbers you have, that should help locate what they are from.
What are you wanting to put them on?
Cheers
Rob