James,
I can’t answer for Day, Falworth, or Mackenzie, but Valdemar Axel Firsoff died in 1981 and his obituary appeared in JBAA vol.92 (1982), no.3, p.139.
RJ Hebbs, could this be Ron Hebbs who ran an astronomical supply company? He traded under the name ‘Bretmain’ from Felixstowe, in Suffolk. He advertised in our journal (eg vol.93 (1982), no.1, p.iii) and in an early edition of the FAS News (https://www.fedastro.org.uk/newsletter/FASnewsolder/FAS_news02.pdf), but this was from a different later address. I seem to remember he stopped trading in the mid to late nineteen-eighties.
I see that this magazine can be found in ‘Google Books’ (https://books.google.co.uk/books/about/Solar_System_Today.html?id=hiDyAAAAMAAJ&redir_esc=y) and the British Library also seem to have it (https://explore.bl.uk/primo_library/libweb/action/search.do;jsessionid=A85336C646EDB4787190BFD9DC1F1DE2?fn=search&ct=search&initialSearch=true&mode=Basic&tab=local_tab&indx=1&dum=true&srt=rank&vid=BLVU1&frbg=&tb=t&vl%28freeText0%29=Solar+system+today&scp.scps=scope%3A%28BLCONTENT%29&vl%28488279563UI0%29=any&vl%28488279563UI0%29=title&vl%28488279563UI0%29=any). In both cases there are multiple references to Hebbs trading address, so I guess I’m right. A search on freeBMD reveals only one Ronald J Hebbs birth in the twentieth century. This was in 1945, so he would be in his late seventies by now, so maybe he is still alive?