1665 Great White insert question 6.22mill

[Magnum44]

This 6.22 mill (79/80) 1665 was purchased from the original owner with an mk4 insert. The first owner for some 36 years could not remeber if the insert was ever replaced at service!

Was this type of mk4 available in 79/80 and possibly original to the watch, or is it a mid-80s insert, and probably a replacement? The pip is tritium and reacts the same as dial and hands to UV light.