From what I read in my books, these seem to be the main changes.
I'm just reading and abridging. Apologies if I got anything wrong.
If you're looking for external differences, they are few until the bubbletop comes along.
P-47D Razorback
-1 Additional lower cowl flaps
-2 Removal of turbocharger shroud
-3 Modifications to fuel and oxygen installations
-4 Provision for water injection
-5 Water injection by engine driven pump, new type turbocharger,
and new centerline shackles for 500lb bomb or fuel drop tank
-6 Minor electrical system changes
-7
-8
-9
-10 R2800-63 engine w/ water injection and new turbocharger design,
also hydraulic system changes and new flaps, and armament modifications
-11 electric water pump, new gun camera design
-12
-13
-14
-15 enlarged forward fuel tank, wing strengthening, new wing racks, fuel delivery redesign,
and jettisonable canopy for pilot egress.
-16 fuel type modifications resulting in increased performance.
-17
-18
-19
-20 R-2800-59 engine with new ignition system, extra ammunition for wing guns,
tail gear strut lengthened, unpainted metal finish
-21 Revised water injection system
-22 New Hamilton Standard Hydromatic paddle blade Props (increased climb rates)
-23 New Curtis Electric prop (increased climb rates)
P-47D-25 Bubbletop
