In general, $20 chips are found at some Baccarat tables -- especially if they have a big table and specially-marked baccarat chips.
With a 5% commission on Banker's hands, each $20 bet equates to a $1 commission ($20 x 5%). However, other casinos, even with Bac chips, just use $25's and above; no $20's (Example: Paris LV).

I don't think Four Queens had big-bac tables during the time this chip was issued. Perhaps it was used on mini-bac tables, but I don't know.

In big poker rooms, $20 chips may be used in poker rooms for use in higher limit games, but not very common. Again, I don't think Four Queens had a separate poker room during this time.