My observation; depends on build date. Later 4-speed cars got the unsilenced housing. A friend's original R/T manual is unsilenced and carries a very late SBD. My CAP auto, with an early SBD has twin snorkles.
Again, just my observation and air cleaners are easy to change as someone already pointed out, but as many 4-speed cars with unsilenced cleaners that I've seen it sure would be one big coincedence.