Primarily Petroliana Shop Talk Forums Shop Talk Petroliana Forums General Petroliana Discussion Age of Sinclair sign

That's neat how you have managed to figure out the third digit. Without doubt you have established that the third digit determines the length of the sign.
I would suggest that these codes were placed there by the manufacturer of the sign and not Sinclair. Following through with that logic. The first digit is probably the customer code i.e. S = Sinclair. The second digit may be a plant or vendor code; it deciphers the location (plant) or who (vendor) made the sign as sign makers did sublet work.