WOW! With the airbox, too, what a find!
The K1/K2 differences are:
-Instruments (the K2 introduced the first "dashboard lites" on the handlebars).
-Rear fender/taillight (K2 got the big one, as did all Hondas in 1972 model year).
-Handlebar switches: the left side switch, if the frame is Old Factory K1, will lack a 'mute' button on the bottom of it for the turn signal beeper, while the New Factory K1 and all K2 had it, and the beeper. The Run/Off switch from the Old Factory (right side) was straight, while the New Factory units have a right-angle lever on them.
-The seat locks on K2, not on K1, and the hinges are different.
-The K1 frame, if it doesn't have them already, needs to have 2 holes in the lower side of the front cradle loop, right ahead of the lower front engine bolsters, and one on each side behind the engine where the vertical(ish) tube joins the lower one(s). This is to let accumulated moisture out, so the frame doesn't rust and rot there. The upper welds on the K1 were not 100% closed, so moisture can work itself into the tubes. The K2 and later ones were 'sealed up' to prevent this in the upper sections.
-The front brake hanger is a "C" shape on Old Factory K1 (1 piece) bikes and a 2-piece arrangement with a "T"-shaped upper part for the pivot pin on New Factory bikes. These parts interchange, though,
Those are the major frame differences, if I didn't forget any.