My Feb 1, 1939 parts book does not have 1557745 listed in the spark plug group. The price section history shows that number-
1555345 says use 1557745 5-1-38
below that it says 1557745 - use 1557746 and 1557747 -10-1-38
There is also a 1557744 that was swirched to the above numbers.
That indicates the two number spark plugs were discontinued in 1939.
The Feb 1, 1940 book has the 1557745 number in the plug group again and is described as a Blue Top #46 - application is 1933-38 (ex CC).Noyt listed for 1939
At one point there was #47 introduced in 1937 but se no farther history of that.
The 1941 book still has the 1557745 plug.
The Nov 1938 Chevrolet Service News describes the changes for 1939 In there it states in the question and answer section "Hoe do the AC-46 spark plugs used in the 1939 engines differ from the Blue Top spark plugs used in the 1938" - answer- The material in the insulation of the new AC-46 spark plugs has been improved so that the plug life is greatly increased over that of the Ac-46 Blue Top. The heat characteristics of the twoplugs are the same"
I believe that there may be more plug info. in the 1939 and 1940 Service News but I haven't
gotten that far yet.
The Chevrolet 216 and 235 engies were always considered "hard on spark plugs"...some brands other than AC would last only a few thousand miles in a Chev and id you were lucky you could get 10,000 from a set of AC.
Todays spark plugs of all brands do much better in these engines.
The little "peanut sized" 10 MM spark plugs used from 1941-48 is long story.