Sorry if this is already common knowledge, I must have re-invented these (someone else must have done them before me but I can't find a list). I'm a newbie on HP but love how adaptable these older calculators are 8^).
On a XYZT calc you can emulate 'drop' by pressing:
'CLX' then '+' (or '-')
I know roll down (Rv) does a similar thing but X goes into T, this way T is preserved and gets copied into Z.
To save battery power press '.' but to restore (nearly) where you were press '+' or '-'(the calc takes '.' as zero so adds zero to Y then drops the stack, so after doing this the X, Y and Z registers are restored).
These all work on an HP67, but I can't find anthing that tells you in the manual 8^(.