The speed sensor retaining screw is 1/4-20 x 5/8" long socket cap head screw, it has a 3/16" hexagon socket. The H-D part number is 3594.

If the cap head screw is replaced with a hexagon head one, it will indeed need a 7/16" wrench.

 

A spanner (wrench) being a larger device will probably make the situation worse not better.

 

This should work....

 

How do they work with your 7/16" A/F bolt?

 

You use a 7/16 open end wrench on the new bolt. I did mine around a year ago so I know it works.

 

Try a little lube, that'll help it slip down in the hole...

 

Perhaps cut a small piece of Allen and glue it to inside of a correct fitting 1/4 inch socket.
The glue keeps allen from falling out but double sided foam tape can work too.
Then use a 1/4 inch wobble tip socket extension.
The wobble tip will give you some angle.
Then use a wobble head 1/4 inch socket wrench.
The above should reduce potential stripping of bolt while providing some increased access.