I have never worked on an 8648A so this is a SWAG, it may be that the attenuator is a HP 33321A series attenuator (4GHz version), this would have the push rods and o-rings.
I have seen some of the lower frequency generators and SA’s that use a completely sealed Wavetek 1.5GHz attenuator (silver block - input/output on the ends, about 0.75” x 0.75” x 2-3”). I have seen these in 50dB and 70dB attenuation versions.
I have never tried to repair bone of these.
Don Bitters
