These are LED replacement bulbs for rear-mounted bayonet sockets commonly found in traditional bell-shaped (directional) reading lights and chartlights.
They are offered in either single-contact or double-contact versions, and warm white and red LED models.
These are the single contact versions.

These are directional bulbs and, therefore, will not work well in dome lights - even though they may have the correct socket.
Safe, cool, reliable and energy efficient.
Red illumination is ideal for chart reading and mitigates vision loss while navigating at night.
Fixtures using the single-contact #1156 BA15s lamps are frequently found in older boats and RVs.
Chart & reading lights in newer boats and RVs typically use the double contact #1142 BA15d; see Related Products, below.
These LED bulbs are designed to work with either 12V or 24V systems.
Replaces GE1156 SC Bay incandescent bulb
B15s - 15 mm single contact base
10 - 30 volts DC
LEDs: (6) high output, surface mounted
Light output: 84 lumens
Beam angle: 120 - optimal for reading and down lights
Dimensions: 1.05" (26.75mm) Dia. x 1.43" (36.3mm) Long
2 year warranty