Gas block with bayonet at the rear position?

[TED]

I SWEAR I saw one of these once but now can not find it despite my google fu. Does such a thing exist and if so why? I imagine it has to do with positioning the bayonet for proper mount with various barrel lengths. But, what specific combination would that matter for?

TED

[ODgreenpizza]

Vickersguide AR-15 Vol. 1 pg. 161-165 shows experimental Colt intermediate length fsb set up to mount bayonet on a 16" barrel