Yes it is and fairly standard on this age bikes although the designs vary
That is interesting, because I've never seen that on another bike, but seeing that photo has jogged a memory of an old grey three speed bike that my dad bought me when I was about nine or ten, I couldn't remember what it was, but the photo reminded me that it had an oil grommet on the BB.
There's a good chance then that the bike I had was a Hercules kids bike. Although I don't remember much about it, I do know that it ended up in the Grand Union Canal near Willesden Junction, much to my regret and shame. ( Don't Ask)