From the 1900's through the 1940's Conn would describe the bore sizes of their cornets and trumpets as "small", "medium" or "large". I have found that these descriptions are very consistent. Here is a table to translate these descriptions into actual bore sizes. You will notice there is no 0.508" bore. As far as I am aware, Conn didn't make any 0.508" bore trombones.
| 1900's - 1940's Conn Trombone Bore Size Descriptions | ||||
| Conn Bore Number | Bore Size Inches | Bore Size Millimeters | Conn's "Artist" Bore size | Conn's "Symphony" Bore size |
|---|---|---|---|---|
| 1½ | 0.458" | 11.64 | Small | - |
| 2½ | 0.484" | 12.30 | Medium | - |
| 3 | 0.500" | 12.70 | Medium-Large | Symphony Small |
| 3¾ | 0.523" | 13.28 | Medium-Large | Symphony Medium |
| 4½ | 0.547" | 13.89 | Large | Symphony Large |
| 5 | 0.562" | 14.27 | Large | - |