In crunching information about the X29 over the years I found the following regarding AB brakes which might be of interest:
1-1939 On this date there were about 25649 X29 built with K brakes in freight service.
6-1939 By this date only 124 out of all X29 built with K brakes had been converted to AB brakes.
1/1/1945 The original deadline for AB brakes required in interchange. The requirement was ultimately extended to 6/30/1953. Very likely using critical war material to replace brake components in ordinary freight cars during the war was not a national priority!
After the war progress in replacement appears to have been very rapid.
11-1952 By this date all but 459 X29 built with K brakes, and in freight service, had been converted to AB brakes.
1-1953 there were 23548 X29 in freight service.
This last date is a few months shy of the 6/30/1953 (final) deadline on which AB brakes would become mandatory for interchange.