Old designation meaning that although type 1 rating is all they claim, it has a gasket on the door, originally just to keep the door from rattling under vibration.
In the MCC world it still exists, but with most mfrs it's now called NEMA 1-G, G for Gasketed. It's that way in MCCs because they only make one unit door type for type 1 and 12, but if you order a NEMA 12 MCC, the other covers are gasketed too, plus the joints are sealed. They changed it from 1A to 1-G because there is also a NEMA type 1A wiring method for MCCs, and people got confused.