Marin
Hi ya, im by no means an expert and will happily stand corrected, but i have an 1989 Muirwoods which im restoring, and found out a few things along the way.
I understand that the zolatone paint and grey and neon colours came later 90/91, so i think you bike is well before this
I have never seen a lugged Marin Frame in fact, the u brake on the bottom bracket would suggest 84- 88.
I have pictures of unlugged Marins without the oversize downtube from 1987-88 and in neon colours.
however i also understand that the oversize downtube which yours looks like it has came in in 88, perhaps the higher end frames were lugged between 86-88.
Beneandemu or WD pro may know more, but its an interesting bike that you have there.
if i had to have a guess i would say 1987 or 86 pre neon years,lol