The 16th edition Radio Handbook (1962) has a "Zero Bias Tetrode Modulator"
featuring a pair of 813s operating as grounded grid, zerio bias modulators
and claims to have 500 watt output. Page 683

I looked in the 11th edition of Radio Handbook and did not find it, but I am
about 87.22% certain that one of the Handbooks features 813s utilized as
triode modulators.

Will keep looking.

Colin K7FM