I’ve been wondering whether the pigeon cipher might be based around bigrams, i.e. where you split the cipher into pairs. If you disregard what seems to be the ‘AOAKN’ key indicator at the start and end, there are two basic ways to split the message into pairs. What is immediately interesting is that if you start immediately after AOAKN, you get three repeated pairs in the message (rather than just one), and that four of those six repeats occur every other pair along a short stretch towards the end of the message.
HV PK DF NF JW YI DD CR QX SR DJ HF PG OV FN MI AP XP AB UZ WY
YN PC MP NW HJ RZ HN LX KG ME MK KO NO IB AK EE QU AO TA RB QR
HD JO FM TP ZE HL KX GH RG GH TJ RZ CQ FN KT QK LD TS GQ IR U
NH VP KD FN FJ WY ID DC RQ XS RD JH FP GO VF NM IA PX PA BU ZW
YY NP CM PN WH JR ZH NL XK GM EM KK ON OI BA KE EQ UA OT AR BQ
RH DJ OF MT PZ EH LK XG HR GG HT JR ZC QF NK TQ KL DT SG QI RU
The WW2 “Slidex” code works by having a table of codes particular to each arm of the armed forces (i.e. Royal Engineers, Operations/Signals, etc) where each bigram pair encodes a particular word often used by that arm. To encipher numbers or words, you bracket them inside a pair of SWITCH ON and SWITCH OFF bigram codes (there are several of each), and pick bigrams that correspond to each letter or digit or digit pair.
Hence I strongly suspect that what we are looking at in “GH RG GH TJ RZ CQ FN” is a sequence of letters enciphered in the Slidex SWITCH mode, i.e. where each letter is enciphered as a bigram.
Unsurprisingly, I would really like to read the 1944 Revised Instructions for using the Slidex R/T Code, and to have copies of the various Slidex code sheets and coordinate offsets used by the Royal Engineers and Infantry Brigade on D/Day, as I suspect these will allow us to read this message (does anyone have copies of these I can see?) Fingers crossed!