I read somewhere on this excellent website before they (the DDR) had an IR line of sight Morse code transmitter in active use too, albeit slightly more recently

I read somewhere on this excellent website before they (the DDR) had an IR line of sight Morse code transmitter in active use too, albeit slightly more recently. https://www.cryptomuseum.com/covert/opto/jo4/index.htm https://www.cryptomuseum.com/covert/opto/jo4/index.htm