[78-L] aluminium discs?

[John Wright]

A collector wrote to me today. Maybe members here can help me advise him 
of what he has:

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The disk is aluminium with a blue material applied to it - like vinyl or 
wax. The disk has a centre hole and an additional hole about 1" away. 
The labels are paper either typed or hand written. All of the disks are 
Jazz recordings dating from the late 1940s to early 50s.  Typical of the 
unusual ones is one featuring Fats Navarro. I've googled it and it's a 
session from NYC 20th September 1949 and features Max Roach amongst 
others.

The same recordings are available on standard 78s.

What is this disk?

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-- 
John Wright