Actually SWG was used since at least the 40's. There's even a chart in it to convert from SWG to thou. I would link it but VCC has closed off public access to their catalogs. I can only assume that someone saw Reynolds was in Birmingham and had an epiphany that the gauges must actually be BWG even though all the Reynolds documentation says SWG when they bother to specify (which is infrequently).

I'm still trying to figure out the exact date Reynolds switched to proper metric instead of calling 0.38mm 0.3mm metric. And of course in French publications, this is 3/10 mes with no mention of SWG/thou.