However, as far as "stop gap" or interim goes what would you define as stop gap?
I see a stop-gap as something that is quickly built to fill in for a more advanced project which will take longer to get into service.
An example fro Spitfire production would be Spitfire Mks VII/VIII and IX.
VII and VIII were designed to take the 60-series Merlins, and had some upgrades for the airframe, such as retractable tail landing gear, as well as additional tanks in the wing leading edge.
But the immediate need to combat the Fw 190A meant a short term solution was needed - so the IX was created. The IX was, basically, the V with the 60-series Merlin installed. There were fewer changes required to get it into production, so it became available about a year earlier than the VIII.
Same thing happened with the Spitfire XIV and 21. The 21 had the new, strengthened, wing was supposed to be the Griffon Spitfire. But the work required to get that into production, was taking too long, so the interim Spitfire XIV was created, using the VIII as a basis.
Naturally, the interim/stop-gap IX and XIV were produced in greater numbers than the VIII and 20-series Spitfires respectively.