Norton Big 4 piston

This is the piston Ken is writing about.  The slit is on the front which is I believe opposite to the drive face.

In the absence of any more informed information from others I can only say that it is not an original B4 piston.  My own 1952 B4 has a Heplex SW10890 piston which is slightly domed and without the slots shown above.

I have however come across pistons with a split skirt similar to the one shown, the photo below being from a Mike Pemberton re-built 19S engine that I have, but never used.  (available for sale should anyone need one).

The slot is to allow expansion of the piston without altering its diameter.

Thanks Peter.  That's set mind at rest.  It seemed odd to have slotted top plus slit skirt.  The slanting angle makes sense.  Otherwise it would tend to leave an upstand witness mark inside the bore.