BA meaning British Association, old-style British thread with numbered diameters; 16BA being the smallest and 0BA the largest. Thread lengths are determined in inches and available in most materials.
They are unusual in that they were probably the most “scientific” design of screw, starting with 0BA at 6.0-mm diameter and 1.0-mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size. They then spoiled this by rounding to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch.
BA Round Slotted Machine Screw