Although it was introduced in the model year 1976, it first appeared in the 1977 Radio Shack catalog on page 13.
A little more info. about the STA-21 Receiver...
Introduced during the '76 model year.
Catalog Number. 31-2071
AMPLIFIER. Power Output at 8ohms: 7 watts per channel, minimum RMS from 20-20,000Hz, with no more than 0.9% total harmonic distortion.
Frequency Response: 15-30,000Hz +/- 2dB.
Signal-to-Noise Ratio: 60dB (Phono), 65dB (Aux).
FM TUNER. Sensitivity (IHF): 2.8uV.
Capture Ratio: 3dB.
Image Rejection: 50dB
Alternate Channel Rejection: 45dB
Stereo Separation: 34dB at 1kHz.
Total Harmonic Distortion: 0.6% stereo; 0.5% mono.
Signal-to-Noise Ratio: 60dB.
AM TUNER. Sensitivity: 200uV for 20dB S+N/N.
Image Rejection: 45dB.
Power Requirements: 120VAC, 60Hz.
Finish: Vinyl Veneer