I found an old booklet in my drawer from 1998 (about 10 years newer than I would have liked).
It does not show a 23D, but it does show 23C a couple times for a couple different trim combinations (blue cloth).
Similarly, no 29K but there is a 29L which is a "secondary color" - also blue. Maybe 29E would be a better example which is also a blue cloth trim code.
The info packet in the GM Heritage Center archive shows code 29 as an available exterior color - Midnight Blue. I do not see a 23 code anywhere for interior or exterior.
K
https://www.gm.com/content/dam/compa...olet-Truck.pdf
https://www.gm.com/content/dam/compa...t-Suburban.pdf