re the 200 gallon steel belly tank or auxiliary tank referred to in the above document
My understanding is that the 200 gallon steel belly tank referred to in the document above is the prototype for what became the 205 gallon 'paper' ferry tank as we are referring to it. Republic made the aerodynamic shape and mechanical function testing prototype(s) out of steel because they worked with steel on a regular basis, and steel was cheap and easy to work with. When the production RFQs were sent out the respondents came up with a couple of different designs - including what became what we (and others) are calling the 205 gallon 'paper' ferry tank, although it was sometimes called a 'fiber' tank. One of the respondents also offered what was referred to as a 'composite' tank made of steel, wood, and plastic. For whatever reason the 'paper' aka 'fiber' tank design was chosen over a steel design - I suspect for weight reasons but do not know.
re the term auxiliary tank
In the pre-war and early-war period the USAAC/AAF often referred to the ferry tanks and DT as auxiliary tanks. The terminology bounced around a bit, and started to use modifiers like droppable, and belly or underwing.
The Australians seemed to use the term belly tank for all drop tanks and ferry tanks if they were not carried in the bomb bays, at least in their contracts.
re the term assemblies
The USAAC/AAF used the term assemblies to denote what are sometimes referred to as knockdown kits by the UK & Australians - ie the parts are shipped to wherever they are going in disassembled or partially disassembled kit form for ease of shipment and reduced space required. It takes a lot less space to package the drop tank half shells when they are packed like spoons and the baffles are still flat pressed stampings. I do not know how many were shipped like this to the ETO but there were a lot of them shipped as knockdown kits to Australia, where the factories would then solder, weld, or braze the appropriate half-shells, baffles, fuel tight fittings, etc. Also, shipping the tanks with all the bits already in place would allow greater chance of damage to the more fragile pieces.
I have also seen the term assemblies used in other military equipment literature to mean assembly kits or knockdown kits.
I do not know if this helps with the discussion, but it provides part of the timeline for the initial use of the British made 108 USgal paper and the US made 205 USgal paper ferry tank relative to the P-47. I think this has already been posted on the forum somewhere (maybe even by me?) in one form or another, but I do not remember for sure.