Musser vibes are the industry standard. I have used the M55 model number because to me they all look the same. Yes sometimes I see gold paint instead of silver, sometimes there are more bits of useless metal getting in the way, but they all work the same. So if it looks like an M55, I call it an M55.
The M55 is a great vibraphone, but like most percussion instruments the design and build quality is sometimes a little dubious. Whilst I have this instrument in my workshop, and before I start doing any work on it, I thought that I would do another buying guide and supplement the first one I did on the Premier 751.