Gents,
I am a model maker and although I deal in small scale, I make the master patterns for kits to be made in resin.
You can search for some of my stuff on the "resicast" site.
As I require reference material, I have quite a bit on the gun and limber.
Scale drawings, photographs etc, plus information on the construction of wheels (spoked).
In my collection is a copy of the manual for the gun.
The conversion from spoked to rubber tyred wheels used the Martin Parry kit, an American design originally used to convert the (ex French WW1) 75mm guns used by the US Army, to be used with rubber tyred wheels. The same was used for the 18pdr guns.....one example of a 4.5" howitzer here in the UK, has the Martin Parry design, with "18 pdr" cast on the body.
If I can be of any help let me know.
georgemoore66 followed by hotmail.co.uk
Only too happy to help if I can.
George.