These are the original Petrie & Lewis Stretch-It Boomerangs! As popular as this trick must have been, they are very hard to find. Except here, of course. Click photos to enlarge.
You know most of the effect. Ask the audience which one is larger/longer. When held like the photo, the bottom one appears longer. You pretend to stretch the "shorter" one and then hold it on the bottom. It now appears longer. Of course, P&L added a surprise ending. Their perfectly constructed gimmick (see photo), allows you to make one of the boomerangs shorter. What a great trick!
Each boomerang is hallmarked twice with the P&L logo and Pat Pend (once on each side). A fine collectible. Comes with photocopy of original instructions.
Price is $115 plus US shipping and insurance.