One of the developers of the DDP-124, William Poduska, who later on became one of the founders of Prime Computer, said in a 2002 interview that the 124 came after the 224, which came after the 24. /wiki/William_Poduska
"DDP-124 Microcircuit General Purpose Digital Computer" (PDF). Confirms the 24, 224, 124 sequence http://www.ddp116.org/products/ddp124/ddp124.pdf
Adams Report 1967, PDF http://ed-thelen.org/comp-hist/AdamsReport1967Q4-1968Q1.pdf
Zhou, Yong (1968). The European Computer Users Handbook 1968/69. Computer Data Series. Computer Consultants Limited (Sixth ed.). Pergamon Press. p. 111.20. ISBN 9781483146690. LCCN 63-25287. 9781483146690
Adams Report 1967, PDF http://ed-thelen.org/comp-hist/AdamsReport1967Q4-1968Q1.pdf
"DDP-24 Announced by Computer Controls". Archive.org. Computers and Automation. July 1965. Retrieved April 1, 2019. https://archive.org/details/bitsavers_computersA_7721632/page/n43
Background, Vol. 7, No. 2 (Aug., 1963), pp. 109-110; published by: Blackwell Publishing on behalf of The International Studies Association
Adrian Wise. "Computer Control Company". Adrian Wise. Retrieved 2008-06-09. http://www.series16.adrianwise.co.uk/history/ccc.html
Bookchin, Murray, (1970), "Toward a Liberatory Technology," in Post-Scarcity Anarchism, AK Press, 2004, ISBN 1-904859-06-2; pp. 57-8 /wiki/ISBN_(identifier)