Wood Window Spiral Balance

I need to replace some early 1950s wood window spiral balances.  Oddly they have silver colored plastic tubes around the springs that have shattered, instead of metal tubes.  They have some metal cleats at the bottom of the spiral that have two screws that screw into the wood sash.  The sash weighs 14 pounds, the tube is 5/8" wide, and the tube is 32 inches tall.  The cleats are riveted to the spiral and cannot be easily removed so would prefer to get them already attached.  Windows are made by The American Wood Window Institute.