The Decca FFSS PU heads from the earliest days of stereo in 1958 where designed to be entirely mono compatible.  That is because the coils were not at 45/45 degrees to the record surface.  One coil was horizontal as it would be in a mono cartridge and the other coil vertical.  So it played a mono record like a mono cartridge.  The downside was slightly less vertical compliance than horizontal compliance.
	
	
		
		
	
	
My Decca cartridge playing the first LP I ever bought, age 8 back in 1955.