The specification of the DEErivative models is mostly as per the the DEE book, but with the following improvements:-

   Frames - Laser cut stainless steel - 1.5mm 304B
     correct shape for loco and tender - except: - cutout for front bogie wheels
   Sprung axleboxes - normally running on bump stops,
                but wheels are pushed into depressions in track
                axleboxes retained by "silhouette" springs
   Bogie - compensated, dummy low-relief springs and compensator bar
   Buffer beams - 1/16" mild steel with etched overlays
   Coupling rods - correct 3D profile
   Scotch crank - eccentric extended to clamp half axles for easy assembly and quartering
   Axle pump - increased to 1/4" diameter
   Drain cocks - disc type, with O-rings to provide both seal and spring
   Lubricator - solid top pattern, directly plumbed into front of cylinder block
            with O-ring seal and no drain outlet - uses a syringe to empty -
            simpler to use, less messy!
   Valve gear - Southern - for r/c reverse
   Boiler - JVR C-type (B-type for SECR D class)
      with electronic LED water level display
   Smokebox - correct shape door - book shows later D1 pattern drawing, but
            photos are correct for D
            optional rivet overlay for later locos
            smokebox rear ring is correct shape
   Safety valve - uses O-ring for seal
             correct shape tubes and base
   Platework - correct shape crank splashers
   Tender - well tank fitted
          no hand pump fitted - supplied with Enots valve and garden spray
          (produces up to 300psi)
   Meths version has sump built into well tank
   Gas tank is bottom feed liquid version with pre-heat loop around boiler backhead
   Wheels - insulated, separate tyres - correct 21 spoke drivers for SECR D class

Added parts (mostly cosmetic):-
   Loco   sprung buffers            Tender   sprung buffers
         coupling hooks                    coupling hooks
         flexible vacuum hoses               flexible vacuum hoses
         lamp brackets                    lamp brackets
         handrails - main, front steps,          handrails - front and rear
             cab side, valve chest cover
         smokebox lubricators (4)            dummy brake stanchion
         dummy clack valves                fire iron hooks
         sandboxes, pipes and filler lids        dummy springs and hangers
         steam reverse pump & control rod      working axlehorns and
                                         correct pattern axleboxes
                                            (SECR omitted)
         brake hangers                    brake hangers
         brake rods                       brake rods
         brake shoes                     brake shoes
         dummy ashpan sides               toolbox doors with handles
         dummy whistle
         handrail stanchions                handrail stanchions
         guard irons                      guard irons
         fall plate and doors between loco and tender
         sliding roof vent

There are many other detail changes to the design as specified in the book -
         sometimes to improve manufacture
         sometimes because I am not limited by manually controlled lathe and mill

The above specification is for the SECR D class 4-4-0. Details will vary for other loco classes, but will generally be to the same standard.