Good spot! It does look like a retractable tailwheel. To the best of my knowledge the Yak-7 doesn't have this feature on all of the different models but I suppose this one might?
Yak-1B from similar timeframe does have a retractable tailwheel, so it could be possible that they adapted that and other aerodynamic improvements to the Yak-7 I guess.
Lucas From Hell posted this picture some months ago, showing a Yak-7 which appears to have the same retractable landing wheel the 3D model for BoK has
I have a question about the Yak-7 I hadn't thought of until now. The radiator in the Yak-7 is considerably larger than on either Yak-1 model that we have and looking at photos I can see that while there were a few different designs... it is indeed a larger radiator system. So that makes me curious... did the Yak-7 for some reason have needs for a more significant radiator system or does it just mean that it will be more capable of keeping the engine cool in hotter regimes?
it's a more "square like" design (a bit less aerodynamic), and it's placed a bit more in the front of the plane. Most profiles you can find in google have the Yak-7 with open radiator and Yak-1 with closed radiator, so that also creates the sensation that it's a bit bigger than what it really is. I took a couple Yak-1 and Yak-7 profiles that look accurate from wings pallete and cropped the open position for the Yak-7 so it's comparable.
There is some difference but it doesn't appear to be that much... since the design came from a trainer couldn't it be that with the Yak-1 part of the radiator is inside the fuselage, while with the Yak-7 having the space for the second canopy they had to move it downwards and forwards?
For example a big part of LaGG's water radiator sits inside the fuselage:
Sadly I couldn't find a similar image showing Yak-1/Yak-7 interior to confirm this.
As a bonus, I made some months ago a picture with the profiles (from wings pallete) of some members of the Yak family at the same scale, good to see the progression in the development of these fighters: