Bristol "Closed contrary to regulations" 1949

The packet below from Bristol to Bath in February 1949, franked with 2½d, was "Closed contrary to regulations" and was charged 5d postage due with a red cachet and a red charge mark.  It was also cancelled with a "BRISTOL / 28 / -*-" undated packet cancel.  One can establish the approximate date from the Bath cancels of the postage due stamps (February 1949).

The 5d postage due charge means that the correct total postal rate (as a sealed packet) would have been 5d rather than the 2½d it was franked with (2½d deficient so 5d postage due charged).  As it was cancelled with an undated cancel I would assume that it was sent as Printed Matter.  From 1940 to 1951 2½d was the rate for 6-8 oz.  The Letter Rate for 6-8 oz was 4d so I can't see why it was charged 5d postage due rather than 3d. 5d was the Letter Rate for 10-12 oz.  Please let me know if you can work it out !