I have checked with my documents. I don't have the specs concerning the 15W, which was discontinued in the 60s.
But you can get some interesting information from the attached link
My advice is that this speaker is nearly 40 years old, and so even if you got the original specs from E-V they would be wrong because of the ageing of the suspension, alnico magnet, etc ! But it remains an outstanding speaker.
TMHO, the smart solution is to make the today's T/S par. measurement of your 15W with modern tools or according the method you can find everywhere on the web, and use them for your enclosure calculation.
Take care,
ERIC

• http://www.geocities.com/tadgesualdo/docpage.html