The M52 was purpose designed and built as a target rifle, heavily constructed and with all the target shooting features available at the time of each version.
The M75 was the junior target rifle, built on the same basic action as the regular 22 sporters in the lineup, just with heavy barrel, prone stock, and peep sights. The main reason the M75 cost less was all it had in common with the M47, 69 and 72 sporters produced in huge numbers. It was probably built with the ordinary (high!) care and quality control of the day while the 52 was made with extreme attention.