I have a ME-50 Expansion Amplifier which I removed from my boat. It was neatly tucked away in a cupboard inside our trawler, so it was never exposed to salt air or spray. I have a paper copy of the manual for it, and have attached a pdf version so you can take a more detailed look if you are interested.
The general description of the unit, from the manual, is as follows:
The ME-50 is a 2 channel audio amplifier capable of delivering up to 25W RMS per channel. Its compact, waterproof design allows for flexible mounting possibilities, and makes it simple to expand the audio system capabilities for any vehicle.
To add additional loudspeakers to a current audio system
To provide amplification for a separate audio zone
As a stand-alone system amplifier for any audio source
Powers up to 4, 4 ohm loudspeakers
Internal distortion limiting circuit prevents harsh distortion Accepts line level, or speaker level input
Polarity switch to optimize interaction with main system
Gain level control to match output to main system
Supports optional external volume control
Waterproof design for marine use
Dimensions: 4.15W x 1.5H x 4.75 D
Shipping: not included; will be shipped USPS from Washington State