Serial Number registry?

[glen]

Model number AN 216
(#5 added in ink, possibly representing 5 lb magnet structure?)
Impedance 16 ohms
Serial number 5101
(January 1951 by my current interpretation of date code)

This Flat-back welded magnet pot would predate the earliest listings I've seen for D216
(first appearance in August 15, 1952 price list, but not yet shown in 1952 catalog on this site)

This would also be earlier than the earliest D208 I've seen,
a flat-back Serial number 302139 that I interpret as being from February 1953, but there is plenty of evidence of earlier D208s:
D208 is shown in 1949 catalog, and included on April 15, 1949 price list.

Anyone seen anything like this before?

Currently listed in ebay auction:

http://cgi.ebay.com/RARE-JBL-JIM-JAM...QQcmdZViewItem