Thanks for all your replies.
When flying aircraft with Transponders with no Alt Encoder then I have always found a sticker on a placard on the transponder display which says 'No Alt Info', or words to that effect. The Cessna that I flew yesterday did not have an encoder on board and had the associated sticker on the Transponder display. Despite that, as a matter of course the selector was set to ALT as a routine procedure.
MG
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When I'm not in my right mind, my left mind gets pretty crowded.