Its package is small enough to fit into a standard envelope.
It looks like this:
It is a circular two stop neutral density filter made of plastic film with a diameter of 25 mm matching the SX-70 lens with a black trim on one side and a self adhesive ring on the other side.
To load 600 film you have to slide a piece of cardboard (i.e.from the film package) under the film pack while you insert it to disable the latch that stops 600 cartridges - see illustrations in the japanese manual (top).
The filter is very thin and does not interfere with the lens movements or with the folding mechanism of the camera.
It covers the distance scale that is printed on the front element of some later SX-70 models.
I have not yet tried to remove the filter and mount it again.
The filter darkens the viewfinder image (guess) by two stops which makes it feel like wearing sunglasses.
This makes focusing somewhat difficult.
Focusing is still relatively easy with the SX-70s equipped with split image rangefinders.
I tried mine with a Sonar which does the focusing by ultrasound.
It allows you to continue using your SX-70 until POLAROID decides to kill 600 film too.
They still sell new cameras that use 600 film but I don't believe that we have another 20 years to go.
The plastic foil filter is of acceptable optical quality but you have to be very careful not to scratch it.
Cleaning it with a cloth is probably enough to scratch it.
If you have access to high quality filter material you can probably make such a filter for yourself.
A coated glass filter would provide better optical quality and be scratch resistant but may be too thick to be left on the lens.
I can recommend the filter method to anyone with a split image circle or Sonar SX-70.
Polaroid should pack those filters into each 600 film pack so they can be replaced before they get scratched.