Looking for a bipod for a Maadi Griffen single shot 50. The shroud has a stub shaft on the end of it that measures .69 in diameter for a bipod to clamp to. I think it originally had a M60 bipod on it?, but would rather not use that.  Anyway, looking for a sturdy one, preferably that folds,  that will mount up to that shaft with the least amount of work. I've looked at several but they never clearly state what mounting system they require. Thanks!

like this one?

Yep! Pretty much exactly! Your bipod looks like it attaches a little different. What mounting system does yours use? On yours, where I see a sling swivel I just have a short section of shaft parallel with the barrel.