I think the RPO code you're looking for is 3B?
like I mentioned before, the GSA vehicles often spec'd specific warnings and placards that would only appear on those vehicles and they don't appear in the 'regular' production code lists..
here is all I could find last time, might have to email one of the vendors that repop these to see if they have seen it before..
good luck!
https://www.crankshaftcoalition.com/...GM_master_list
B34 H.D. Front Floor Mat
B35 H.D. Rear Floor Mat
B37 Colored floor mats
B39 Load Floor Carpet
B30 2367 P FLC R COVERING FLOOR CARPET 910508 E T E A
B31 2368 P FLC R COVERING FLOOR VINYL, FRT, FULL WIDTH 980624 E T E A
B32 2369 P FMF R COVERING FRT FLOOR MATS, AUX 880729 E E E A
B33 2370 P FMR R COVERING REAR FLOOR MATS, AUX 880729 E E E A
B34 2371 P FMF R COVERING FRT FLOOR MATS, CARPETED INSERT 880729 E E E A
B35 2372 P FMR R COVERING REAR FLOOR MATS, CARPETED INSERT 900212 E E E A
B36 2373 P COVERING FLOOR MAT, LUGG COMPT 840223 E E E A
B37 2374 P COVERING FLOOR MAT, FRT & RR, AUX 970508 E E E A
B38 2375 P FLC R COVERING FLOOR VINYL, FRT & RR, FULL WIDTH 980624 E T E A
B39 2376 P COVERING FLOOR CARPET, LOAD FLOOR 830101 E E E A
B3D 2344 P EQUIPMENT SCHOOL BUS 860408 E T T A
B3E 2345 P NOISE CONTROL 80DB NOISE PASS-BY JURISDICTION 980618 P 1 1 A
B3H 2348 C COVERING FLOOR CARPET, LOAD FLOOR - DELETE 980618 E 1 1 A
B3J 2350 P EQUIPMENT DIESEL 860408 E T T A
B3L 2352 P TIR M TIRE ALL P155/80R13/N BL R/PE ST TL ALS 880411 P 1 1 A
B3M 2353 P EQUIPMENT SCHOOL BUS, DELUXE 860408 E T T A
B3N 2354 P COVERING FLOOR MAT, FRT & RR, AUX - NOT DESIRED 20205 E T E A