It seems strange these should break as there is no stress put on them when fitted. I can only think that stress would come from the long stock bolt being loose which would then allow leverage to be applied to the long tang.
Baz
I suppose if you banged the guard accidentally from the side, that would put stress on the sharp bend down to the tang? But it would have to be brittle to break there.
Vintage Airguns Gallery
..Above link posted with permission from Gareth W-B In British slang an anorak is a person who has a very strong interest in niche subjects.