The serial number is 5266. According to the amp tech who restored it, it's a Marshall '1974' 18w 1x12. These were made from '66-'68 and are apparently quite sought after.
I haven't found a lot of info, other than that Marshall are currently re-issuing this very model.
. Also, that there appears to be an active cloning community.
A friend of mine gave this amp to me a couple of years ago. I told him that it looked like an old Marshall and was therefore probably quite valuable. He still gave it to me, for which I am eternally grateful to him, since it's the best amp I've ever played. He, in turn, found it in the dumps and brought it home. In case you're wondering, I'm not going to sell this baby - at least not for any reasonable amount of money. I love it too much and it would be disrespectful to my friend.
Bonus question: Are there any well-known guitarists who were/are known to use these amps?