Have you looked into the parts on MikesXS?
Wow, you have put me to the test on this one!! I didn't know what you talked about, so I first looked in the "parts for sale" for a user called MikesXS, then eBay for the same user, and finally googled for "MikesXS" and found MikesXS.com....

No, there is nothing like this switch there. Actually, I have some XS400 searches on eBay and every handlebar switch of the Yamaha bike is different from this on my Sanglas bike. What is more, the engine type in the Sanglas bikes was "2W2", that seems a unique designation for this engine, even when it's a carbon copy of the "2J0" and others that now my memory can't recall. Instruments were not Denso but Veglia, cast wheels were spanish Akront etc.
As you can see in the picture, the switch is "sandwiched" in the lever and master cylinder assembly. If you remove the fixing bolts the switch retracts and the wires and solders are exposed. If I'm not wrong the master cylinder is Magura, but I recall the switches are Denso -from what I've read in Sanglas forums-, but I don't have the proof nor have I seen any brand name stamped or etched in them.
My only hope is that some of you can remember this type of switch in any of the bikes you have had along your biking career.