Occasionally, someone notices that the Bureau Number marked on some Navy aircraft is followed by a letter, usually lower case, and wonders what that is all about. This practice dates back to at least the late 1950s.
U.S. Navy aircraft were usually purchased in annual batches. There were almost always changes (improvements, problem fixes, different avionics, etc.) from batch to batch or even within a batch, which meant that there were groups of aircraft that were slightly different configurations, but not so different as to justify a change in the designation dash number. From a manufacturing configuration control standpoint, these were referred to as blocks. The maintenance documents provided to the Navy provided a cross reference of Bureau Numbers for each block for initial configuration definition and subsequent control. For convenience, the blocks were designated by letters, with A being assigned to the first one, B to the second, and so forth. Although not the usual practice in these matters, the letter "O" was not skipped for the 14th block as shown here.
However, aircraft were updated and improved during the periodic overhauls and aircraft from the same batch did not necessarily go through overhaul at the same time. Since changes continued throughout the life of the aircraft, that meant that the configuration of aircraft that had been produced in a particular block began to diverge from others in that block. As a result, the block designation letter was not as meaningful from a configuration definition standpoint after the first overhaul or so and not necessarily put back on when the aircraft was repainted, as on this post-1962-designation-change F3H.
Also, not all manufacturers seem to have followed this practice. However, it was common on those from Douglas and McDonnell.