It's the transformer that has the detector coil in it, the one that's coupled to the detector, normally an untuned primary connected in the anode circuit of the RF amp and often with a tapped (for band change) secondary (tuneable) It's the one in the photo that has a 220pF coming from it that goes to the grid of the detector valve, It can also be seen in the 3rd photo in post#4.
The thing that concerns me about the AL1 conversion is that the output transformer might not be the optimum load and that the total external "cathode" resistance is at least 850 ohms and is not fully bypassed frequency wise, the manufactures valve data gives 350 ohms for the usual typical HT etc:
https://frank.pocnet.net/sheets/030/a/AL1.pdf
Lawrence.