So this was inspired by someone on DA's thing that I saw a few years ago that was about a similar premise but I can't for the life of me find it! If I or someone else ever finds it I'll link it here.
This was also inspired by some dragons in HTTYD who seem to have this sort of thing. The ones I can name right now are the Bewilderbeast (most obvious), Speed Stinger, and Hideous Zippleback.
So since Evolution doesn't like inventing new parts from nothing to often (especially something complex like a limb) and flying lizards like the Draco exist why not modify the ribs (or in Coelurosauravus' case "Dermal Bones"
) to make Pseudo Limbs? This way Dragons of all kinds (Western, Wyvern, serpentine, etc) are still technically Tetrapods and not (or used to be) Hexapods... which other then insects don't exist... as far as we know
Ribs tend to be easy for evolution to make though which is why there's so much variation between tetrapods (four limbed animals with a back bone).
So what I did was take a gliding lizard similar to the extinct ones and over the course of millions of years took their hinged ribs and moved them to make a gliding wing (Like in 3 and 4) while moving the struts and other ribs forward to support the front half of the torso.
Over the course of evolution though some bones thicken and change until they start resembling a limb and one of the ribs in the torso thickens and starts creating a crude socket for one of these bones. Creating a pseudo scapula. The sternum also starts to thicken as primitive flight muscles start to form and attach to the thickened bones.
At the end the ribs morph into a pseudo limb and the lizards, now dragons have powered flight. Of coarse depending on how many bones you start with (in this case 7) some may become similar to wrist joints or joints in the fingers etc. For this one I think that the "fingers" would have small muscles and ligaments to bend and flex the bone and maybe cartilage as the animal is flying or when it comes to folding the wing... Or the "fingers" could be entirely cartilage while the actual bones form the limb and "wrist" joints...
I don't know, just a fun thought experiment.