My 28 is a regular old plain bbl field grade. My dad wasn't a rib guy and so I never became on altho I did have a mat rib A5 Browning for a while.
Where was the 176th located? That unit sounds familiar so I may have flown joint missions with them? The 281st was in Nha Trang and flew all over II Korps and a bit in the south edge of I Korps. Primary purpose in life was to support MAC V SOG altho we also worked with the SF Mike Forces.
All my Model 12's are field grade with the exception og the 28. The 176th was in Chu Lai, I Corp and then I went to 7/1 ACS in Vihn Long, IV Corp, Crewchief