Those big four are always the last four of the BuNo on the tail. As for knowing what the first ones were, that's just a matter of knowing the system. The age of E-2s means it was going to be no earlier that 153---, but since I'm not home to check my book of E-2 numbers, I just took a logical guess that it was 163--- and went to Baugher's site to confirm that number and there it was.
Actually, when you understand the U.S. military system, it's just as simple as the Canadian versioin. Unless you start looking at the stuff of things prior to the late 1920s and then that can be real interesting!
