From the picture, looks like the old Russian favorite the LPO-50:
http://waffen-der-welt.alices-world....fer/index.html
http://www.russianwarrior.com/STMMai...on_flame.htm&1
Apparently fielded in the late 60s-early 70s in USSR, license-produced in China, so it was probably quite new for the war with Vietnam. Say maybe 5 years earlier than the existing unit you mention, barring any real info.
I can hardly believe no older models were in service before that though.