It is. I was going to reply with the other name it was known by - 'Slip-wing Hurricane'. The idea was to create lift and range, with the upper wing being 'slipped', or jettisoned, when ready for action. Didn't get very far ...
Heck, it was a guess, having seen the gun camera port on the '17 before! And I'd forgotten you'd posted pics taken recently at Bourgas, even though I replied to your thread!
I'll post something later tonight.