This circuit was originally published in SPRAT back in Winter 1987.
The use of Neosid toroids was simply because these were available at the time. Today I would used T37-6 (yellow) cores instead. Also the PIN diode switch is a bit of overkill, again used as I had some. 1N4148 diodes will probably be as good or change the TX-RX break-in to use the simpler arrangement normally used with sub-1W QRP rigs.
My very first contact on this rig was around 4000 miles to an SSB station in Chicago. I only had a crystal in the SSB part of the band at the time. I heard him working someone so tail-ended on CW and got, "QRZ the station on CW". A solid QSO resulted, the first of many more once I got a 21.060MHz crystal. Offset between TX and RX was around 600-800Hz and so no special IRT offset was needed. The rig was in use for several years and was, at one time, the main rig used out of choice because it was small and a bundle of fun. Sadly the rig was dismantled to make other rigs and no photo exists, at least not that I can find. If I do find one I will add it here.
I have included the original SPRAT articles as images. These show the circuit, the description and parts list. Sorry about the hand drawn schematic, but this was the 1980s!