Digging a little deeper into the Service Manual, the output attenuator assy (& Reverse Power Protection- RPP) are quite different between the 1GHz -A and the -B, -C & -D higher frequency models.
For the -A 1GHz model, the attenuator & RPP module is A7 with p/n 08647-61803.
For the -B, C, & D models, the attenuator is A11 with p/n 33222-60011 while the separate RPP module has p/n? 08648-60025.
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Both the -A (1GHz) and B,C,D (2, 3 & 4 GHz) models are shown in the Service Manual drawings as "130dB attenuators in 10dB steps".
The B,C,D models use the 33222 attenuator which uses plungers & O-rings but the -A version possibly uses a simpler attenuator construction (ie no O-rings or plungers) and incorporates the RPP circuitry, so it has a different p/n.?
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UPDATE: it seems the 10dB increments also 'glitch' at the 50 & 60dB settings. I can't hear any mechanical clicks when the attenuator encoder is changed (with the covers on), so do I have a dicky/dirty/ encoder ?? (I'd still like to know how the sub-decade attenuator levels are achieved !? ?The plot thickens.
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