Hi Roger et al.
First, thanks to Dave King for preserving Col Nelson's photos. I met the gentlemen a couple of times and he was always a source of interesting tales. I wish I had had a chance to discuss the photos with him when he was alive... we are losing so much of our corporate memory. More incentive to caption my own photos!
Roger; from the point of view of an Instructor-in-Gunnery, I would like to add some observations re: the SP in question:
As you noted there was no useful role for the 40mm on a carrier... it could not keep up with convoys, had no ammunition storage (AA guns eat ammo at a prodigious rate) and it would suffer from terrible inaccuracy; four successful shots at a stationary target at 600 yards is hardly an adequate test of accuracy. Without some method to lock out the suspension or stabilise the gun platform (outriggers, leveling pads, etc) the gun wouldn't hit a canary going 3 mph at 200 inches!
Imagine the work out for the gunners if the platform wasn't level; cranking that gun around is hard work, especially uphill... maintaining an accurate track while cranking uphill and trying to control the downhill swing a 180 degrees later would be murder.
For any useful AA capability the SP would have to have a fast "coming into action" timing... less than one minute would suffice at WWII engagement ranges. This requires a large well trained detachment (the artillery have "detachments", the Navy and armoured corps have "crews"). The truck mounted Bofors could carry a full detachment of seven, the SP carrier looks like four would be a pinch.
Picture, if you will, a Lloyd carrier burdened with a 1 ton gun, add on four burley soldiers and basic kit (another 1/2 a ton) and then slap on a trailer with 1 ton of ammunition and all the gun stores and personal kit of the detachment. Maybe we add another carrier with just ammo and stores and three more men... the truck version was a better solution. For armoured formations the tank mounted version was much more useful as it had onboard hydraulics, however, it still suffered from leveling problems.
The right decision was made to scrap the 40mm Lloyd SP.
Hope this helps with your research.
By the way, is the David gun part of your book?
Cheers! Mike