http://militaryrussia.ru/blog/topic-602.html
It is very detailed site about russian equipement and weapons.
Use automatic translation of page to english.
"adopted in 1974 as standard equipment"
MG - 74 is carried by this subs (refering to this site)
- pr.641
- 877/636
- pr.671RTM / 671RTMK
- pr.971