In practical terms what does the Prefix on ICs usually mean?
In 4000-series it primarily indicates the manufacturer as Andy says : though I thought CD4xxx was originally RCA, then adopted by (some) other manufacturers. 

This is in contrast to 74-series where the later letters in the prefix (74S, 74LS, 74F, and later 74HC or 74HCT etc) indicated mainly technological differences. (though 74F was originally proprietary to Fairchild it also meant Fast indicating a non-standard process)

So you can usually accomplish the logical aspects of the design from a single datasheet. 

However - and especially with a monostable - the analog aspects are likely to vary between manufacturers. Maximum and minimum supply voltages, leakage currents, speed are likely to differ; so for example. if you are using unusually high resistances for unusually long delays, using the correct datasheet is essential.

I remember the HEF4000 series as significantly different in speed and supply voltage ranges, so possibly deserve to be treated as a different logic family, as do the crossover 74HC[T]4xxx with TTL-compatible supply voltages and speeds. However the logical design and pinout are generally the same, so within mutually acceptable voltage and speed ranges, they would normally be interchangeable. (Need to dig out some old databooks to crosscheck this, so don't take it as definitive)