This should help... Theoretically MkIV's came into effect at kit ser # 1000 BUT it was more like 1100---have the details somewhere. Anyway....some work for you ---tee hee heee. Does it have the Taildragger gear legs with the step? Does it have a 1/1/2" Tailspring necked down to take a MAULE Tailwheel? Does it have Elevator with inset trim Tab? Does it have Round Rudder? Does it have Dorsal steel stringer---i.e Fin tube goes to the T'Deck FIN NOT OFFSET Does it have a baggage locker? Does it have Gas strut fixings just behind left shoulder in fuse frame? Does it have Triangular steel welded gussets in EACH corner of the "quarterlight window" one gusset is Seat Belt Anchor point. Does it have Fiberglass seat? Does it have Spring Loaded Door latches? Does it have Gusset from Top rear carry thru tube, L&R, outside the fuse side frame, welded to the rear Spar Bolt attach bushing---a bit like a shelf bracket. Does it have two Strap like Plates welded on Rear Wing Carry thru tube to inside of rear door post? Are ends of Wing Carry thru tubes welded closed? Does it have two plates for Fuel Tap behind Vernier Throttle mounting plate? Are there "glue plates" welded at rear of fuse where inspection plate could be fitted for theelevator Push Pull Tube Clevis Fork end they Go from Fuse to Fin Spar. Are fuselage tubes at this point sort of splayed /opened out for greater Bolt clearance? There are a couple of other subtle points that can't be checked unless tubes cut -- as Avid upgraded some Tube wall thicknesses to get Gross up to 1150/1200lbs. BUT These should keep you busy for a short while--- and Send everybody with a "MKIV" running to check!!! heee heee.. Have fun, Regards, Steve at Airdale... Also... Looking at our log book (Airdale's) it shows your SN 1388C as a C Model STOL TD. The serial numbers before, back to about SN 1025 are listed as MKIVs but with a D after the number (as your's has a C). The serial numbers after yours are also listed as D's with the designation of MkIV etc. Hope this doesn't muddy the water more. Jerry at Airdale