I've been contacted by a gentleman who needs help with the M-number of this multipart. Says he can see Franklin marking. The measurements are as follows: 10.5W (outer top shell) x 7.5 (middle shell) x 3 (inner shell) x 7.5 (height). From photo is looks to be radio treated. Does anyone know if Franklin is known to "radio treat" their insulators?

Looking thru the book the closest is M-2755 by Franklin or Jeffrey-Dewitt. thanks Dustin for confirmation.