To the best of my knowledge, the 123GT was made in 67, 68 and 69 and
most had the B18B engine rated at 115 SAE Horsepower. They had a tach on
the top of the dash, driving lights on the bumper and a dual-path braking
system. I suspect they may have had stiffer springs and shocks than the
standard 122, fancier seats, etc but they were basically a fancy 122.

I seriously doubt that yours had a B21 engine because the B21 didn't come
out until 1975 and was not an easy fit in the 122/123.
BTW the actual model numbers were:
123 (all 2-door sedans) model 133
122 2 door sedan model 132
121 2 door sedan model 131
122 wagons model 222
121 wagons model 221
122 4 door sedan model 122
121 4-door sedan model 121
Early ones and especially ones sold in Europe also known as "Amazon".
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George Downs Bartlesville, Heart of the USA!