Doug,
As Al photos show, a 20mm Thunderbolt mount was a PT mount used in a limited number of boats. There were several iterations that it went thru with the final version being 4 20mm canons on the mount.
Many are available on Shapeways here:
https://www.shapeways.com/marketplace?type=product&q=pt+thunderbolt

Hi Mirangi,
I have a print of the John Lambert (1986) drawing of various twin 20mm mounts.
They were produced using info direct from the Royal Navy Ordnance Museum, Priddy's Hard, Gosport.
Also shown are details of the individual guns, to 1/24 scale with dimensions of all parts😀.
It is a single A1 sheet, but I can scan it onto four A3 sheets if you wish.
These are the drawings that I used to produce the two twin 20mm mounts for my 1/72 H class destroyer, HMS Hotspur. Like most things in my workshop - not yet completed🤔

I've never heard of a 'Thunderbolt mount' for these guns. Please clarify.
For me a Thunderbolt is a P47 fighter bomber. Carried all sorts of nasties, but never 20mm guns as far as I know.
Cheers, Doug😎

Interiors showing the mount with and without the MK14 gunsight.

I don't know if this site will help you?https://www.ibiblio.org/hyperwar/USN/ref/PT-Manual/index.html

Appreciate all your help, great deal of knowledge out there.

Surprised at how difficult to get info on these guns, considering how many have been in service around the world.

We had them on two of the ships I served on in the 1970's but i dont even have any photos of them, fun to fire them though !!!!

Morning.
A few pictures of which there are numerous on line but I can’t find any dimensions as yet.
Will keep looking.
Regards Bill

I have quite a few photos of the interior of the mount if you need more detail.

I managed to download a 3d file from the net some time ago.
That should give you some help. I can always help

I can scan drawings from "Allied Coastal Forces of World War II, Volume 1" which has extensive drawings of Oerlikons. However the drawings do no have dimensions. That said, there is a scale included with the drawings so that you can scale the images.
I'll need your e-mail because they are copy righted and I do not want to post the images.
Please send me a PM if you want to go this route.