All Glenfield 60's are the old style, Marlin 60's prior to 1981 or 1982 are considered the old style. The Glenfield name was dropped in 1982, so any Marlin 60 after 1982 would be a new style.
The feed ramp is a different design on the new style and around 1983 or 1984 the last shot hold open feature was added. I have an '83 without and an '84 with, but have seen reports of others from '83 with LSHO. I believe the trigger guard is slightly different on the old vs new style.
The barrel length was shortened in the late 80's (maybe 1988) to 15 rounds to comply with some New Jersey law and then in the early 2000s the barrel length was shortened to match the magazine length.
The first 2 digits of the serial number will tell you the year of manufacture. Take the first 2 and subtract from 100 and you get the year. My 1983 s/n is 17xxxxxx, my 1984 s/n is 16xxxxxx, my 1998 s/n is 02xxxxxx.
So for parts, what year is your gun? With 18 round tube, it's older than 1998ish.