Regarding Jeff's comment, I was of the impression that the signal was
generated at a high level prior to the attenuator, and attenuator
settings were correct for the 512-1024 band as well as the others.
Since your "8640B in a can" has the 512-1024 switch position, it should
be possible to install a home-made doubler in the same space where the
HP doubler would go, prior to the attenuator.

Fred
K4DII

The doubler is an external device that screws onto the front panel output
socket, there is no provision for putting it internally prior to the
attenuator. The 512-1024 switch position merely changes the displayed
frequency.

Jeff