Hi Hilary,
I have had a look at the policemen of Hull, The first number 228 was issued to George Watson on 29th March 1883 and from there we have :- George Henry Clift issued 31,7,1909. then Edward Marson on 1.11.1910, then Edward Stephenson 11,12,1923 and lastly James Harry Curtis 26.5.1952 I have nothing in my books for after 1974 but looking at their dress I would say they were well before then.
As for PC 378 I have William Warcup issued 13.8,1908 then Harry Edward Lee issued 16.6,1936 then John Michael Marsh issued 28.1,1963 and Kenneth George Wood issued 29.11.1963
In the East Riding for PC 228 I found David Eltherington issued 26.1.1948 and Walter Gerald Kingston issued 4.4.1949, There were no PC 378 shown.
In the East Riding there was a George Plaxton joined 7.7,1872 but number not known.
Hope this helps Rockofages