For reference... here are all the ones I've seen. Some of the bottom ones I have only seen used by Mecum.
SSP - Single-Sided Porcelain
DSP - Double-Sided Porcelain
SST - Single-Sided Tin
DST - Double-Sided Tin
PP - Pump plate
PPP - Porcelain Pump Plate
SSPN - Single-Sided Porcelain Neon
DSPN - Double-Sided Porcelain Neon
SSTN - Single-Sided Tin Neon
DSTN - Double-Sided Tin Neon
SSN - Single-Sided Neon
DSN - Double-Sided Neon
SSL - Single-Sided Light-Up Sign
DSL - Double-Sided Light-Up Sign
E - Embossed
Flange - 90 degree Bend Double Sided
