it is an air filter and water separator for your 110v air compressor that is underneath it that services your air system when the engine is not running. it should either have a drain valve or petcock on the bottom or have an automatic drain.
Thank you, I can stop wondering about that one now.
By the way Tom, the picture you posted and heads up on the "collection of hoses" and "y's" saved me finding some damaged things in that bay in the spring. You guys are great!
Bill