Hmmm... I've just done an image search and it could be the Asperitas which has only recently, in 2017, been added to the International Cloud Atlas no less. My photo & a comparison example from the Cloud Appreciation Society
Oh good lord no... it's on much higher level of international dullness all of it's own making. And one that is strangely compelling For example, have you have checked out the visual spectacle of the Nimbostratus page?
Not sure what Triumph this is and also not quite sure how the exhaust header is fixed into the head. A nice little DOT
I'm not sure... I've only ever heard the term 'sloper' used with Panthers and always thought it was refering to the slope of the barrel. But prompted by your question a web search seems to imply it was a generic term. And the wiki for a BSA sloper mentions "Its engine rhythm, together with its easy handling may have added to choice of its name."
Speedway motorcycle engines also come in both upright and sloper. There was a lovely unrestored Panther many moons ago in a transport yard I briefly worked at a lovely looking machine.