Here is a group I posted many years ago on the USMF.
It all belonged to a good friend who was a Special Forces sniper instructor in El Salvador, and are his bring backs.
Here is some info Nkomo added in the original thread:
First off, when I looked at the red shield in the grouping, I thought it was a 6. I was wrong. It is a B. I have tracked down that specific patch to the Ramon Belloso BIRI (Immediate Reaction Infantry Battalion). This unit was specifically formed to fight a counter-insurgency war against the the Marxist FMLN. This unit was disbanded during the 1992 Chapultepec Peace Accords.
According to what I have found, the US Special Forces started training the Ramon Belloso BIRI in 1982. The USSF was instrumental in setting up and training these counter-insurgency units. They were very effective and they became a target of propaganda for left-wing groups in El Salvador and other places around the world (especially in the United States).
I am not 100% sure which military zone that this BIRI operated out of, but I am thinking that it was the Chalatenango District. If that is the case, it would have fallen under the FMLN's Northern Front of operations called the Apolinario Serrano Northern Front. Furthermore, the FMLN group that this USSF soldier and the Ramon Belloso BIRI would have fought against would have been specifically the FMLN-FPL.