"could it be the same function as our 'push-button' which gives actual altitude rather than relative displacement?"
Sounds like it, BOAC.
You have a similar system on your 747-400's, but the ABS (or "F/L") switch appears to be momentary and won't allow the system to remain in that display mode, so I imagine this is not something you would use a lot(?).
Rgds.
NSEU.