I've a feeling
Telephone Lines used to stock them - as just a bell without a case, in fact I think that's where I bought the one I have, but I checked their catalogue following Dave's initial post I couldn't find one. It may be worth contacting him, as I've always found him very helpful - details are on the "contact us" page of the web site. And yes, it
does fit inside a 232.
Otherwise, all I can suggest is that they do occasionally turn up on eBay, so patience is the order of the day. The alternative is to do the job the original way and mount a bellset 26 (or a bell 59A, given that you are using the capacitor in the master socket) underneath.
Regarding your line cord, I notice that you have transposed the white and red leads when compared with the original cord. As I suggested above, would transposing the leads on the original cord have the same effect?