“This is a very British thread.”
True, but then it is about British tubing.
It’s also really about a very short period (generally’83 to ‘89) in UK (in reality just English?) and world bicycle manufacturing.
Indeed a pivotal and transitional period.
As mentioned earlier in this thread (and elsewhere) various small and medium framebuilders and bike manufacturers took an interest in ‘mountain bikes’
Notably Overburys, Saracen (particularly for F W Evans), Ridgeback, Claud Butler and Dawes. (Raleigh watched for a while - as it often did - later on with BMX.)
At that time frames - from the cheapest to the most expensive - used lugs.
Conventionally and routinely this led to horizontal crossbars (now commonly called toptubes!)
Early British MTBs copied (literally) American ones. Which one/s has been the subject of speculation on this forum…
Design development happened quite rapidly (mostly in California). Frames became shorter and, more significantly, often had sloping toptubes.
This resulted in quandaries and difficulties in the UK.
Less so in the US which had ‘discovered’ Taiwan and tig welding.
Unfortunately 531 wasn’t suitable for tig welding.
Consumer ‘tastes’ also changed and American/jJapanese/Taiwanese designed/manufactured bikes became ‘desirable’.
Marketing played a part of course. CroMo became a ‘thing’.