I was just about to make a similar comment. The close-up pic shows the panel line running through the 'patch', and there are other darker marks on the lifting hole on the second and third aircraft in pic 2, post 67.
Either the hole was covered to prevent dirt entering, or perhaps it made a heck of a whistling noise in flight, if left open !
Looking at it again, I'd say this temporary cover is hurriedly cut, thin fabric, doped in place almost certainly with the same fabric and dope used to cover gun ports, the dope acting as the adhesive. It might even be thin cloth or paper, maybe the cotton '4 x 2' used to clean gun barrels for example, probably originally white, where the dope has made it semi- translucent, hence the panel line appearing through the material.