I think I can help clear this up.
This amalgam of the early 70--73 parts manuals shows left switch assembly part #3, and the myriad of part numbers used for that switch.
Here's pics of the undersides of my (2) '71 XS1Bs, for both the early and late versions. Note that the 306 XS2 switch (frosting black) first appeared on the later '71 XS1B. Also note that they are both embossed with "YHA205".
This shows that the '71 switches have 2mm (0.080") thick lever arms.
From previous threads on these switches, I feel very confident that all the 70-73 left handlebar switches are mechanically identical, the only differences being:
- Color of the body (aluminum or frosting black)
- Paint color used for the lettering and hack marks (black, white, red, none)
- Possible differences in the wire sheathing and connectors.
In summary, I'm very confident that the On/Off switch pieces are identical and interchangeable within the 70-73 switch model years. Same with the "mirror-imaged" Lo/Hi switch pieces...