As luck would have it, here's a Pentax 1/21 post I made yesterday containing a link to a searchable copy of the full 100-page user manual for this meter.

On manual page 30 (PDF page 34) is the following:

"The 1/21 meter is powered by two batteries - one 1.3 volt mercury battery and one 9 volt dry cell. The mercury battery operates the high range, 'H', and the dry cell operates the low range, 'L.'"