Dr. Honeydew wrote:
"We have a new signal generator here at the Labs, but it came without
documentation,"

Terman elaborates on Thevenin`s theorem on page 74 of his 1955 opus.
From Fig. 3-19 (b) on page 75, I would suggest loading the generator
with resistance until its open-circuit voltage is cut in half. That
resistance should equal the internal impedance of the generator which
you have already determined is resistive. Provided you don`t maintain
that sort of overload too long on your 1-watt source, you should do no
damage to it.

Best regards, Richard Harrison, KB5WZI