Just acquired a 1985 Riviera GT, s/n WE 07661.

I realize these serial numbers aren't inherently significant like some serial numbering schemes, yet I do suspect there's patterns in the numerical portion. Here's why: the importer orders various quantities of bikes with certain specs, for example a quantity of 125 blue Riviera GT's with a 53 cm frame size, another 125 red Riviera GT's with the 53 cm frame size, perhaps 100 blue Riviera GT's with a 50 cm frame size, etc etc for all the various models that are being completed under that order. So the manufacturer isn't going to randomly make exactly (a quantity of ) one bike of one size/frame/color, then another completely different size/frame/color, then hey the next minute let's make yet another size/frame/color. That would not be efficient from a manufacturing standpoint. The tubing cutter is instead going to cut all of the downtubes for one particular frame size at one time, rather than changing back and forth to cut different random lengths; the welder/brazer is going to set up a jig and weld a whole bunch of the exact same frame, rather than randomly changing back and forth between different setups. The paint booth also likes to paint with the same color for as long as possible. So I suspect that the serial number sequence is still random--but possibly in chunks according to whatever was on the production ticket for that day or week. Of course, they may have had multiple lines bringing completed bikes through for serial number assignments, so you might have one or two numbers in a row for type A (line #1 's production for a few minutes) then one or two numbers for type B (line 2's production for a few minutes) then back to type A for a few more sequential numbers...then back to type B...Still sort of random but within the constraints of producing a large numbers of similar bikes in roughly the same time period.