When that radio was made, a "Tune" and "Load" adjustment were typically accessible from the rear panel. The tune should be peaked for carrier, and the Load control for best modulated power.
If it's peaked properly a peak reading meter should show something closer to 11 or 12 Watts modulated.
And an average-reading meter will show more or less what you're seeing.
Unless something in there has worn out or gone bad in the last 50-plus years. Pretty sure those folks closed down in the late 1960s.
Just exactly how much of it should work like new after five-plus decades is hard to predict. Some of it? All of it?
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