Never saw this one before but it cannot be a 1950s musclebike because the musclebike era didn't start until 1963.
The 1950s did spawn the pre musclebike Pig Bikes, but the seat is at least mid 1960s as are the handlebars.
That said, I don't worry about the value because I rarely sell or part with a bike once I get it unless it it is something I ended up with in a trade or I built one for a neighborhood kid. Thats why I only have a few bikes. Well, that and the fact that I am not yet independently wealthy.
I would consider buying it and if I wanted it bad enough it could be worth $300 because the frame is cool.
I am researching the frame and if I can find something on it I will let you know