While making my daily scan of Gunbroker.com I can across this interesting Costa Mesa AR-180.
I had seen it before and dismissed it as a mixmaster gun with a Howa lower or it was using a former AR-18 lower.
You will notice the markings around the safety selector and the fact that the safety stops would allow for 180 degrees of movement.
I was able to figure out the serial from this picture as S3803.
I had to blow it up but I believe it is legit.
I surmise that the Costa Mesa plant was using up all the parts on hand and they used a lower that was originally intended for an AR-18 on this gun. This makes sense when you consider that the last Cost Mesa AR-180 serial was S4067 so this was probably built in the last month of production.