As I mentioned in my previous post, Breyers haven’t always been made of Tenite Acetate. I’m not talking about the Stablemates and Little Bit/Paddock Pals, both of whom made the transition to ABS (acrylonitrile-butadiene-styrene) some time ago. There have been a couple of times in the past when Breyer used other types of Cellulose Acetate.
The first known incidence of this is from the early to mid-1970s: the Chalky Era. Since Tenite was expensive, and in short supply, Breyer went overseas to purchase the off-brand Cellulose Acetate they needed to continue production. It might not have been "Tenite," but it was still the same stuff chemically - more or less. It’s not the color this stuff came in that made the difference: the pigments used to color Cellulose Acetate have little effect on the plastic’s basic properties. It’s the formula itself: every manufacturer has their own, subtly different recipe for Cellulose Acetate.
Because each brand of Cellulose Acetate is slightly different chemically, it’s generally not a good idea to start mixing them together - just like it’s not a good idea to mix different brands of the same kind of paint together. So the brittleness some Chalkies are known for may not just be a result of a different "recipe," it could also be the result of a slightly incompatible regrind mix.
In the late 1970s, Breyer again was forced, due to cost, to switch to a different type of Cellulose Acetate. This time it was Cellulose Acetate Propionate (CAP.) CAP is most definitely not compatible with plain old Cellulose Acetate: the literature from the Eastman Chemical makes it very clear that mixing the two is a very bad idea. (Oddly, it can be mixed, to some degree, with the third form of Tenite - Cellulose Acetate Butyrate, or CAB.)
The "B" mold mark was added to molds during this time period - ca. 1979-1982 - to help distinguish the Propionate-molded models from the Acetate-molded ones, and prevent regrinding accidents. It’s assumed that the "B" mark was added to most Breyer molds in production during that time period - but we don’t know for certain. I’ve been casually keeping track of that data point just to confirm or verify. And who knows? Maybe a surprise or two will show up. (I keep track of all sorts of crazy data points. It’s a wonder I get anything done at all!)
But why did they use a "B" mark - shouldn’t it have been "P" for Propionate, instead? I wonder if they originally decided to go with the Butyrate form of Tenite instead, and only switched to the Propionate at the last minute. The molding properties of the Butyrate form of Tenite are more similar to the Acetate than the Propionate.
It’s probably only a coincidence - maybe it was just a "B for Breyer" thing, or a random mold stamp they picked up. Or maybe it was just an artifact of their internal production process: they may have simplified things for the workers by referring to the two different plastics by letter, rather than by name. If Acetate was "Plastic A," then Propionate could have become "Plastic B." No clue.
In general, both Propionate and Butyrate are more dimensionally stable, and can hold detail better than Acetate. However, these properties come at a price: less flexibility. That lack of flexibility results in this sort of thing happening more often:
A lot could have been done to manipulate Propionate and Butyrate to behave more like Acetate - either by changing the plastic formula (adding or subtracting plasticizers) or recalibrating the molding process (manipulating the heat, the pressure, or the injection speed.) It must have been too much of a hassle: they opted to go back to Acetate once it was feasible to do so.
While most molds had the B mold mark removed by 1982, it lingered on for years afterwards on a small number of molds, most notably the Rearing Stallion. Some molds seem to be more rare than others with the B mark, but I don’t know of many hobbyists who collect them the way, say, Chalkies are collected.
As far as I know, Traditional-scale Breyers are still made from Cellulose Acetate - though I am unsure if it is specifically the Tenite brand. I haven’t seen the word "Tenite" in any of the promotional literature lately, but that doesn’t necessarily mean anything. As the saying goes, the absence of evidence isn’t evidence of absence.