Would this be what became the XM177/M4?

The XM177 was introduced in 1960s, long predating the M16A2 carbine which was developed in the 1980s.

The M4 development started only a tiny bit later than the M16A2 carbine development, as a requested improvement on the XM177. It was originally known as the XM177E2, then renamed the XM4, and then finally the M4.

The XM4s are basically identical to M16A2 carbines. The later standardized on M4s finally moved to a flat top. I believe a very small number of carbines with receivers marked "M4" (rather than "XM4") but retaining the fixed carry handle were pushed out early to the US Army Rangers, but I can't find the source I read that on.

Ah ok that makes sense. I never heard of the M16A2 carbine, so I figured it fit somewhere into the M4's lineage. The question was where though lol. Thanks for the info!

I think it was essentially a parallel design that quickly became redundant, and then when the M4 went flattop, the M16A2 carbine became not just redundant but obsolete.