6th,13th, 20th and the last pre-war issue 27th April 1940, Volume 12, issue 312.
There were 312 issues before printing was suspended because of paper shortages during WWII therefore 27th April 1940 (issue 312) is the last until the new format started in 1954. Below the notification that from 4th May practical motorist will be incorporated into Practical Engineering for the time being and a cover from that period.
Although the pre-war magazines were not published after 1940 encyclopedia's continued, shown below 1941 5th edition and 1946 6th edition.