Those are "flight time" tests of powders made with different charcoals.
The author measured the time of flight of a film canister as a means to measure the strength of a powder.
The 1st number refers to a footnote, the X refers to a column on a chart, while the 2nd number is the actual flight time, and then the date of the test.
Details about the testing and the chart itself are on this page where the data came from:--->>> http://www.wichitabuggywhip.com/fire...oal_tests.html