The BA15d type incandescent bulbs are used in many sailboats, and sometimes in  RVs in in some reading lamps.
Note that these are NOT the BAY15d bulbs which have asymetric ears. In that series the case is ground and the two contacts provide two sources of power.

These bulbs are also used externally in automotive tail lights. However, the LED equivalents do not make good tail lights.

The base of a BA15d bulb is neutral and is a metallic 15mm diameter base with symmetric bayonet side-pins. Two contacts at the bottom provide a contact for the ground and positive leads.

Typical bulb numbers for this group are the 68, 90, 94, 1004, 1076, and 1142 whose physical dimensions are

     width 1.0 inches
     length 2.0 inches

The 1004 bulb uses the same base but measures

     width 0.75 inches
     length 1.8 inches

See donsbulbs.com for detailed specifications of the bulb you are replacing.