HP-21

The first of the 20 series was essentially an HP-35 in a smaller, less expensive package. Since the HP-21 had 33% more ROM than the HP-35 it had room for a few extra features, namely:
Trig functions in radian mode. (The mode was selected by a slide switch.)
Display formatting. (DSP # displayed # decimal places in scientific mode. DSP . # displayed # decimal places in fixed mode.)
Polar/rectangular conversions.
Storage arithmetic. This was implemented using special keys for M+, M-, Mx and M/.
10^x.