A couple of P-47 questions...

1) When were dorsal fins (fillets?) introduced in production P-47D's?
I know they've been installed in P-47's as Field Modification Kits
but, though I've gone through some books and websites, I can't get to
a conclusion about when they started to be fitted to P-47's in the
production line.

2) Plus: were there any other external major differences between the
-28 and -30 other than the "dive brakes" ("compressibility brakes"?
What's the correct name?) and the repositioning of the landing lights
due to installation of the dive brakes?

Tks,

Vicente