Never owned either, but I do see that the 950 and 990 use the same core mechanism, so the counter and secondary belts might be the same but maybe someone else will know. The flywheel belt (82-385-214-21) is the same on both. Although it's not on any cross-reference lists, I do know the original is 3mm wide, and one person used a 10" belt with success, so
FBS9.9 should work. The closest PRB belt I could find is FRX10.1, so that may be what the original was.
For the TPR-990, the counter belt (82-530-210-01) is
SBR4.6 and the two secondary belts (82-564-244-01) are in the PRB reference as SCX4.0, or the equivalent
SBS4.0.