I checked a couple of old ball joints I have and also my selection of different sized ball joint boots.
The ball joint I have measures the same:
48.2 mm to bottom of groove and 19.3 mm across the bottom of the taper pin.
I have a new boot (part of a selection, not specific to the Dolly upper ball joint). This measures 38.5mm on the lip that sits in the groove and 14 mm on the top hole.
You need to remember rubber stretches quite easily and quite a lot!!!
I tried it on the upper ball joint and it is a very nice fit - a bit of a stretch on the bottom but once on it doesn't want to slip off and that is with out any clips or cable ties.
Any new boots I have used on any joints have always been smaller than the actual joint and stretch over. This ensures a good seal. Use ones that measure the same size and you run the risk of dirt getting in because when you fit the wire/clip/cable tie it will "ruffle up" possible allowing small gaps to which will allow moisture and dirt to get in.
Finally, I would trust what the club is saying, i.e. their ones fit! - £25 a year for a club that has good quality spares as well as a decent forum is not a lot to pay - the magazine is not a bad read either.
I have attached pics I took this afternoon - the fitted boot is the one I measured.