I also thought that you couldn't use a six speed freewheel on a 120 axle, but you are holding the evidence in your arms, and so there you be! Suntour made a narrow 6 that fit in the space of a 5, but you said you tested two different freewheels.

It is really no big deal to put a 126 into a 120 space. Also no big deal to put a 130 axle (126 OLD) into an older hub...dish only loses 3 mm, so there is great flexibility between 120 setups and 126 setups (the difference being about one wheel washer moved from one side to the other). It may also be that the frame was set for 126 at some time in the past...did you measure that dimension?

If the wheel doesn't just fall in the wheel is probably a 126.