They had MO since they began all the way through the last year I believe. Which I found out the serial numbers for the late 70's to the 90's are not known to be able to date them unlike the early 70's & older. Most Murray's starting in the late 70's to late 80's had put on black with white lettering decals above the bottom bracket with where they were manufactured in Lawrenceburg, Tennessee then the month & year.
They used the same sprocket design through most of the companies life except on some models like the little kids bikes and the 10 speeds & such.
I noticed in the 1 pic you posted the serial number. The stem which was made by Wald has numbers on it that I can see which shows 8074 so maybe it's from 1974 or maybe 1980 but usually the last 2 numbers are the year.