[Collins] mechanical filter insertion loss what about it?

[Claudio Marchesini]

Hello,
I have a 75A-4 with standard F455J-31 (3,1 kc/s BW) SSB filter where I have
added two filters (the F455N-20     526-9163-00    2.1 @ 6 dB and the
F455N-40     526-9160-00    4.0 @ 6 dB) coming from an old 390A's IF
chassis.
It seems to me that the insertion loss of the 3,1 kc/s is very high if
compared with the others (they are perfectly matched...), I have measured 10
dB of difference (tracking generator+spectrum analyzer on IF).
Anybody says if this IL is correct for this filter or not? My be damaged?
Thank you very much in advance.
Greetings.

Claudio