Since no one else is giving you a bump, I will put in my 0.02 cents. Could not quite follow all your explanation.
The piece on the left looks like a piece of the shift rod bushing (#56). Mine had not broken, but had started to turn to a similar color as yours. Replacement is semi-transparent white. Inside diameter is just under one inch.
Are you sure the piece on the right is not part of the shift rod bushing also? Looks like the same color, hard to determine the material from the pics though.
Other thought on the piece on the right. On this site, I think one of the replacement coupler bushings is white (part of #61). Could the piece on the right be that bushing that has deteriorated like the shift rod bushing? Whoever had replaced my coupler had left the old bushing parts on the bottom of the tunnel and the TOP (!?!?!) of the coupler.
