There is one of these on fleabay at the present time. Looks like a beauty. It is a 1975 and i have a similar 6 string i believe is a '71. They seem prevelant in the northwest, i see alot for sale around here (which makes sense since they were imported into Portland). These guitars are hard to date but are early -mid 70's as far as i know. I doubt you'll find the exact year.
Top end Lyle acoustics are totally amazing and as good as any Martin from the same era as far as i'm concerned. It's funny to me how their value is so incredibly low. I put mine up against a '75 Martin a month ago and i swear the Lyle blew it away. Maybe the snobs will figure it out someday, but lets hope not. Sssshhhhhhhhhhhhhhhhh.
p.s. yours is a top of the line Lyle.