Every DEC component part has a 2-5-2 number. Sometimes it also has another designator, like an M-series module number*. In the 2-5-2 numbering system the first 2 digits denote the part class (for example, 23 - ROMS/PROMS/EPROMS, 29 - vendor parts), the middle 5 digits denote the unique part, and the last 2 digits denote a particular variation. Note that not all 2-5-2 part "numbers" are numeric - things like manuals are EK-series (EK was originally "Education Services- hardcopy").
*Note: technically, M-series parts are 00-Mnnnn-00, where the 0's don't display. An RQDX1, M8639-YA, is 00-M8639-YA, but it also has other 2-5-2 part numbers (there should be a 54-series etched PCB number, for example)
Explanation provided by Terry Kennedy.