Jan 1931 comes from a more precise source than Eddie Shaw - the late Frank Andrews handwritten notebooks covering the whole series month by month. Eddie's DATES should only be used a guide.
1930 ! Not 1931 ! Please correct the date.
According to the late Eddie Shaw's D.A.T.E.S. numbers up to R.874 were issued in 1930. As you may discover, both sides must have been recorded in 1930 (which is possibly the more relevant).