Because I ran into glaring defects in two out of two Inrad #758 filters (2.1KHz 6.3MHz i.f.), I am posting this warning here. One of the #758 filters I purchased new in 2015 from Inrad and the other #758 filter I purchased used. I suspect a large percentage of the Inrad #758 filters in the field have this defect - they are actually much wider than the 2.1KHz spec, measuring about 2.3KHz wide.
Below is a spectrum analysis I took of an Inrad #758 2.1KHz filter compared with an Inrad #757 2.4KHz filter. There isn't much difference between them - perhaps the 2.1KHz filter is 50hz narrower. These spectra were taken with the filters installed in an Omni VI+ (thus the abbreviated low end response due to dsp). In any event, with the same center frequency and PBT setting, the 2.1KHz filter should show high freq cutoff 150hz lower, (2400-2100)/2, than the 2.4KHz filter.
I suggest you proceed with caution when considering buying one of these filters.
Inrad readily accepted return of the new #758 filter. But when they didn't respond to my request for a replacement that met specs, I accepted their offer for a refund. Seven months later I bought a used #758 filter and found that it had the same problem and was again able to return the filter.
Update: I managed to buy an older Inrad #758 2.1KHz filter (identifiable by label with small print) that performed properly.
Here's the spectra of the first Inrad #758 purchased from Inrad:
And here's the spectra of the used Inrad #758 purchased 7 months later, looking awfully similar: