64M154359
This unit would be a 1964. The first two numbers denote the year. We know this because our unit had a sheet glued to the inside of the upper cabinet that had a diagram of the camper and some basic information. It appeared to have come from the factory. The year marked there matched the first two numbers in our serial number on the underside.
I'm afraid I don't know what, if anything, the rest of it means. And so far, I haven't found anyone else that does, either. If you happened to work at the factory building Mobile Scouts back in the 1950's and 1960's, you'd make my day if you got in touch with me! I have a blue-billion questions to ask you.
So grab a flashlight, and get on your back under your unit and see what you can find! And if you have a photo of your serial number, send it in so I can show people what to look for and where!