Hello TJ,
If you're still looking for more detail on the Mark XV quad 20-mm Thunderbolt, there is some info in "Building, Detailing & Converting the 1:35 Scale Italeri PT-596" by T. Garth Connelly. Pages 34 to 42 cover the conversion of the standard kit to PT-559 and include a drawing and photos of the original and the model's Thunderbolt installation. The pages don't scan very well, but if you're interested I can post you photocopies.
I see, with some surprise, that 'Thunderbolt' is painted in quite large text on the front shield of the gun, at least on a photo of PT-559 in Sept 1944.
(If the Ilfracombe MBC is active any time over the Xmas period, between 23rd - 27th, I may be able to visit the Lake for a chat with the lads.)
Keep sailing,
Mike B