Here's the info from my uncompleted seatbelt article (just was working on it again, that's good!)
Location Standard Deluxe
Front lap, bucket seat 237 137
Center lap, bench seat 235 131
Front shoulder 277 (coupe)177 (coupe)
? (convertible) ? (convertible)
Rear lap 231 131
A one or two digit number indicating the production week, a single letter (apparently related to which plant (or possibly production line) produced the part), and the two-digit calendar year number.