Need Help Identifing Speaker Cabinet Model

I know this is the Fender forum, but I have not been able to locate a Standel forum. Does anyone by chance know what type of Standel cabinet this is?

As you can see the upper right hand plate emblem is missing. It has a JBL D131 or D131F speaker inside of it and if it is a D131 the serial number roughly matches ones produced around 1963 or 1964. My guess is it is an Imperial model something, but if anyone knows anything about these cabinets it would be nice to know when I list it for sale.

http://www.gohome.50webs.com/IMG_1449.JPG
http://www.gohome.50webs.com/IMG_1450.JPG
http://www.gohome.50webs.com/IMG_1453.JPG
http://www.gohome.50webs.com/IMG_1452.JPG

Imperial XV?

I've seen Imperial IV, Imperial XII, and Imperial XV Standel cabinets, so I was hoping someone knew the difference between them, or if it just depended on the year it was made.

my completely WAG (wild ass guess) is IV, XII, and XV might have stood for speaker size (4, 12 and 15 respectively)

so maybe it's a XII? take that with a grain (or three) of salt :D