Need to see it closer. Definately a 350 or 500cc OHV single of Brit design. Not Triumph for it is not a vertical twin as Triumph were from '38 & on. Probably a BSA, still it could be a Norton, not a Royal Enfield, Velocette, Panther or such. Primary chain case is pressed steel & some of the bikes like Triumph & RE had one piece alloy ones.
The bike standing on its rear stand points to a spring loaded rear stand so many were using including AJS & Matchless. You see there were more then 2 or 3 Brit m/c makers & no it is not a Panther.
The bun pad behind the saddle was installed by a prior owner & the rack behind that is not stock. Looking more & more like a BSA.
Of interest the support bar to the rear part of the front mudguard could ALSO be brought down as a front stand when working on the f/end of the bike. Battery holder & rear chain guard are typically Brit.
I was in the m/c business of selling & servicing Brit irons in '47 though also owned my own Brit iron or two in '46. So bike dates back to my time.