[MrBungle]

The order probably doesn’t make a lot of difference here. I experimented with this recently and found no practical difference if there was less than about 50Hz unless you’re building narrow CW filters. This probably explains why they’re so expensive. I got through 200 crystals (thank you aliexpress!) before I had a good set for a CW filter!

The matching is the most important thing for flatness and insertion loss rather than overall shape. It’s pretty difficult to do this in circuit so I bought a NanoVNA for £30 off eBay and build filters to match 50 ohms using matching transformers (just two FT37-43). I keep everything 50 ohms - so much easier! freedom to fiddle with the thing in isolation then.

I would in your case attempt to work out the optimum filter impedance out of circuit then build matching transformers for the Plessey ICs based on their impedance spec. No idea what that is. NE602 is about 1.5K.

Edit: to note you need crystals with the same series and parallel resonant frequency for really effective filters. I worked out I couldn’t be bothered to go that far so stuck with two at 500Hz and 1.2Khz ones for my CW receiver.