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.'"
"When making a portrait, my approach is quite the same as when I am portraying a rock. I do not wish to impose my personality upon the sitter, but, keeping myself open to receive reactions from his own special ego, record this with nothing added: except of course when I am working professionally, when money enters in,—then for a price, I become a liar..."
— Edward Weston, Daybooks, Vol. II, February 2, 1932