I went back to my old notes (had to go find them, I've been lazily using a copy of my own book for a long time), and it's not 1cm (10mm), but 4-5mm (more like 1/2 cm).
Here's what I have, from 2 K0 frames and a K1, K2 and K4 frame:
Carbs and hoses are 65mm each.
The K1, K2 and K4 were 65mm from the carb's hose flange (where the hose is flush to the carb) to the center of the 6mm bolt hole that hangs their airbox.
The K0 bikes were both longer at 69mm and 70mm (2 different bikes, one was near-sandcast) from the face of the hose mount on the carbs' intake flange (not much of a flange) to the center of the 6mm hanger bolt for the airbox.
I measured the K0 bikes and the K1 at Brown's Sport Cycle circa 1970 using a metric tape measure and by looking thru the mounting hole(s) of the airboxes to estimate the end-of-distance (the 0 of the tape measure), noting that the values were different between the K0 and K1. The K2 was/is my own, and measures like the K1 notes I had. The reason for the measuring at the time was the perception that the 'new' K1 airbox didn't "breathe as well", having smaller slots, so someone wanted to know if he could buy an earlier one and fit it to his K1.
I measured the K4 somewhere around 1974 when I worked part-time at Littleton Honda, using [possibly] more questionable hardware: IIRC I used a long aluminum rod thru the mounting holes for the airbox of the K4 and then measured the diameter of that rod, divided by 2, to get it's center dimension. Then I used a (plastic) set of calipers between the rod's side and the hose-stop edge of the #4 carb, converting those readings to metric (it was a cheap set of inch calipers).
The take-away I've always had from this was/is: the distance between the back of the carbs of the K0 bikes to the mounting hole is different, a little bit further. What I did NOT measure, though, and which may have been smarter(?) was: how far forward of the mid-frame downtube is the mounting tab's hole? That might be more consistent?
