I was looking at the drawing by Jim Gianco N51B and Jerry Haigwood W51H
The output attenuator is two 150 Ohm and one 39 Ohm R.
The source is ERA-3+ which has a bias resistor of 270 Ohm.
Essentially since the output impedance of the MMIC is rather high the shunt of 270 Ohm prevails so this attenuater has one side fed from source impedance of 270 ohms and looking in from the output side the impedance calculates to 71 Ohm.
this is (270 || 150) +39) || (150) Ohm.
Return loss on output of ERA3 is >quoted at 21db at 1 GHz so if I assume output impedance of
then ( (270 || 50 ||150) +39 || 150) = 63 ohms.
I think there is another output type too and I think a quick mental calculation showed that one to be 73 Ohm but I will check a post a little later.
I might be wrong
( Was AP2AM. Haven't got new CS yet))