Ed wrote:
Please educate me. I see one of these for sale on ebay that plugs into a
crystal socket. I know that a lot of boatanchor IF's are 455Kc, does this
generate the IF for old receivers, or is it a bandpass filter?


There's an interesting-looking Hallicrafters 455kc crystal filter
on eBay, I assume that's what Ed is referring to.
The crystal holder has a neat lead seal with a Hallicrafters "h"
impressed on it, to keep anyone from opening the holder.

That type of filter is a prewar design (one-crystal filter),
where the crystal was part of a balanced circuit
before the first I.F. amplifier. It could be used
with a variable "phasing" capacitor
to null a heterodyne, or sharpen reception bandwidth.
Most receivers used a switch-selected set of resistors
across the single crystal filter
to reduce the selectivity to permit 'phone reception.
(for example: the Collins 75A-2 receiver)

Better than nothing on a crowded c.w. band, but the ringing
is annoying and tiring. Modern crystal filters
are the lattice circuit type, where an array of crystals are used
to get a flat-topped passband of the desired bandwidth.
A "six pole crystal filter" has six individual crystals in the filter.

73,
Ed Knobloch