Progress so far: Forks assembled today and starting to connect the wiring in the nacelle.
The fork bush retaining cup (widget) was very easy to fit and everything went together well, slid the oil seal holders on stanchions, then retainer cups, shims, top bearings, bottom bearings, clamped stanchions in the yokes to tighten bottom nut, PTFE around fork leg threads and a couple of turns of string in the grove and screwed it together with an oil seal holder tool.
When I ordered the stainless steel oil seal holders from Barleycorn Engineering Simon said he'll be honest and warn me that some have had a problem with them not screwing on their BSA's fork legs. He told me he’s invested a lot of money in a business partnership with a multi million pound manufacturing company near him to manufacture them, only to have problems with the threads not being the right size for some fork legs. With some there's been no problem and happy BSA owners. Puzzled and concerned by this variation, he got together with the company he's been working with and they have carried out some tests and investigations and discovered there is a variation of seven thou in the thread diameter on bikes made by BSA in the mid 60's to the ones made in the early 50s. I can't remember if he said early 50's were the smaller or the other way around.
With my agreement he has used the oil seal holders I bought from him as a test (that’s why it’s taken so long for me to get them). His idea was to open up the thread slightly to try to eliminate the problem by making 'a one size fits all' as it's better for them to screw on easily than being too tight or not fit at all. As it seems to be successful in my case his plans are to do the same for the others he has.
(Edit): Referring to the above: Is this why this seller is making such a bold statement in the description?
https://www.ebay.co.uk/itm/BSA-CHROME-FORK-OIL-SEAL-HOLDERS-BSA-A-B-C-models-29-5310-Made-in-ENGLAND-S-S/282536850999?hash=item41c8823637:g:dXUAAOSwPYZU8X~C