What function(s) do these resistors play in this heater circuit?

[mort]

Fuse the heaters? I just had dinner so I may be slow, but seems to me that the failure current of a 2-3 short is from one side or the other - or both - of the 6v to ground, the fuse in a heater circuit is not to ground but instead is in series with the 6v., and so would not open in these cases.

As per the schematic, this one is fused and you are correct, it did not blow.

[Leo_Gnardo]

As per the schematic, this one is fused and you are correct, it did not blow.

Filament fuses never do, except at the worst possible moment. Like when you're powerin' up for a big audition, TV appearance, royal command performance, big paying gig, like that. Useless as a screen door on a submarine.

[Chuck H]

Useless as a screen door on a submarine.

Hey! My submarine HAS a screen door.

It's nice for letting fresh air in and keeping flies out when I'm not submerged