They would have started to appear in the late 80s, that's when noticeable numbers of Ads began appearing.I was there in Spring- summer '81 and the only captured rifles that I saw while w/ Atllacatl @ Sitio de Niño were Israeli Galil's and M16's. Serial no. revealed from Nicaragua (Somoza regime) and given to FMLN by Danny Ortega/ Sandinistas.
I'm sure the "G"s had many different type rifles but only remember reports of G3s and FALs.
Only Communist Block stuff that I remember seeing/ having was Czech C-4 explosive and RPG rockets.
I don't know about late 80's.