I've seen a few of those BSA sporters over the years, including a nice between-the-wars 'High Power' (IIRC) and currently own a post-WWII Model E sporter. None of them had a rear sight anything like the example above!

With vintage Brit hardware, the rules are: "There are no rules" !

Also, there has been at least one thread on these forums with pics of commercial P-14 sporters. Would be worth posting the link here.