If they weren't suitable for 230V use, then HP wouldn't have fitted these connectors to equipment they either made in the UK or imported during the 1960's.
I suspect the power cords were never made in Europe and HP sourced them all from the US, hence they are only marked 110V. They changed to using the IEC connector in the 1970's, if they had carried on using the PH163 connector then I'm sure they would have had all the relevant European approvals & markings.
There are some available in Germany (where the mains wall plug is non polarised/reversible) here;
https://www.ebay.de/itm/222745856688 , their not cheap, but the only other way to find one is to buy something you probably don't want to get one.
P.S. You should check the condition of any original power cords & determine they are safe to use, the same as you would for any other cord (including modern ones).
David