Hi Scott,
Got your beads today and they are made from polystyrene. Probably high impact polystyrene (HIPS) - which has some rubber added to increase toughness. The rubber also makes the material opaque.

As far as age & place of origin - if they just appeared on the market recently, I'd guess China. If you have some time, you could search on alibaba for plastic beads and see if you can locate similar ones.

A bit of plastics history-
Polystyrene was a commercialized in 1931 in Germany by the I.G. Farben company. The Bakelite Corp. also made polystyrene - here is a quote from a 1943 text, "Handbook of Plastics" p. 209:

"Natural Bakelite polystyrene is a crystal-clear, water-white material. It is also supplied in a wide range of beautiful translucent and transparent colors for unusual effects in product and package design".

I love how the word "natural" is applied to a plastic...

I'm not sure when HIPS became widely available - probably in the 1950-1960 timeframe. But I don't think your beads are that old. HIPS has very poor wear characteristics so if these had been used for decades in Africa, I'd expect them to be pitted and worn. Here is an image of some HIPS beads that seem to have been around the block a few times. The seller tried to convince me they were Bakelite, and I think is still listing them on line as Bakelite. Of course, maybe they are Bakelite polystyrene! I occasionally see whole strands of these light, lemony yellow HIPS beads with the African trade bead sellers. Wish I knew where they got them! They are the same size & aspect ratio as the short, oblate phenolic resin beads that also come out of the African trade. I've also gotten a few of them mixed in with phenolic beads.

Anyway, good luck with your bead research - shall I mail the beads back to you?