Caswell #39 question

I've been googling what exactly the Caswell #39 (or #36, etc) mod is exactly.

It is obviously featured on a number of JF mods, so I would like some clarity. It seems to be an extra gain stage via pull (brahma #39 or switch 1959RR custom/platinum and others).

Was the original Tim Caswell mod just an extra gain stage or is there a voicing change going on as well? And does this go for JF mods too?

I thought the 2/3 switch sounded the same on my 59RR Platinum but with low/high gain changes. For the Brahma #39, does the pull-pot activate more gain or change the tone too?

I am curious because the brahma #39 (without extra gain activated) is a hot-rodded plexi sound and the slash AFD sound is a bright JCM-800 tone (with the extra gain activated) so I was wondering if there was an actual tone change there or just more gain.

Any clarification is greatly appreciated!

I am sure Rob will chime in but I will add my personal experience with the Brahma.

The JF Brahma I have is an early one. Possibly 2009? I had it updated in late 2011 but still pre-#39 version.

When I pull on the gain knob to engage the boost (or whatever it is) I get more bottom and gain. There is some other type of mid range warmth that is more pronounced when pulled.

This is my cold dead hands module. All others are judged against it and will continue to be.

Rob,
Let's hear from you on this one.