EIA Code: 137 (The first three digits stamped on the square back).
CTS was a massive supplier of audio drivers for classic guitar amplifiers (like vintage Fender and Ampeg amps) and high-fidelity systems from the 1950s through the 1970s.
Production Date: Mid-June 1976 (or potentially 1966)
Date Code: 616. The digit 6 indicates the year ending in 6 (most likely 1976 given the frame and ceramic build style), and 16 indicates it was produced during the 16th week of the year (around mid-April).
Part Number: 985-003563
This is the specific OEM (Original Equipment Manufacturer) customer tracking number or internal part number used to fulfill orders for a specific brand's cabinet or amplifier.