[OG68]

From the Corvette Forum;
The easiest was to ID the date of the radio is to examine the big output transistor (DS-501) on the back of the radio in the heat sink area (unless someone has changed it). There are four numbers on that transistor - first two are the year, last two are the week of the year.

7532- Transistor made first week of August (32nd week) of 1975. Which would make it a 76/77 model as HR said.