Beck, Robert Mark (30 December 1960). "PB-250 - A High Speed Serial General Purpose Computer Using Magnetostrictive Delay Line Storage". Managing Requirements Knowledge, International Workshop On(AFIPS): 284–285, 287. doi:10.1109/afips.1960.58. The first production computer was delivered in October 1960.
ed-thelen.org
Weik, Martin H. (Mar 1961). "PACKARD BELL 250". ed-thelen.org. A Third Survey of Domestic Electronic Digital Computing Systems.