In the electrical field, "T" means tubular, and the number is diameter in 1/8" increments. That's for flourescent lamps anyways. Maybe this is something similar. I also read the the 194 is 3.25. I reckon the wedge base is the same. My eyes tell me that this cluster bulb is a little smaller that the ones I put in the DIC yesterday, which were 24's from O'Reilly's. But I have to order some sockets too.