possible castes
A caste system, to me, implies a relatively stagnant society--if things are changing a lot, then people can see change, and look to take advantage of it. Hence, I'll assume that it's 1920's tech, but not all that dynamic.
There could be a caste of coal heavers.miners/other physical labor. Children can be useful in these trades from the time they're very small--so they have no education, or even skills useful for other menial trades.
Engineers, in the sence of people that keep engines running, could be an arcane tradition pased on from father to son, might be a fairly powerful caste, as it keeps the heavy things running. Without their skill and muscle, not a single wheel can turn...
Anything complex can be its own caste, and the castes are interdependent. It's the handing down of knowlege, incomprehensible to other groups, that would allow organizations to become castes, cemented in time by traditions.
Priests, of course, can be their own caste, and if the god/devil/pantheon, supports castes (or, more likely, is proclaimed to do so by the priests) they'll be a very high caste indeed.
The caste of "Readers," might be critical--they're a sub-caste of the priestly or government (Or both) that read proposed books and articles for wrong ideas.
The longer the caste system is, the more likley it is to be broken down into more and more subcastes.
I would expect that there would need to be no significant external threat, becuase a caste system can be very wasteful of talent.