Richard,
FWIW the Wingnut Kit has a plywood panel there, I just checked my completed one and there it is. The color call out is for Dark Grey or Light Wood so I'd guess the linen version could be inferred from the former color. Obviously like many others different airframes had a variety of differences on the smaller details, in this case your photo doesn't lie!
Lovely work so far, this Kit needs more effort then the competition's version but you are more then meeting that challenge!
Cheers,
Lance