The spreader plates are flat metal pieces that keep the two vertical weigh guide rods from bowing apart and allowing the counterweights to come loose. Best Practice is to insert a spreader plate every two feet in the weight stack. To calculate how many spreader plates your arbors should be equipped with just take the height in inches between upper and lower arbor frames and divide by 24.
Many stages have a spreader plate that is painted RED inserted just above the ballast weight (aka: "pipe weight") to provide a visual reminder to the weight loading crew that they should stop removing weights at this point.
USITT Stage Equipment Expo in Charlotte, North Carolina, Steven Rees, owner of Graybeard Solutions, introduced the Retrospreader™. It is a two-piece assembly that can be installed on an existing counterweight arbor without disassembling it. Steven has done his homework on this item and it has been stress-tested and patented.
The Retrospreader™ can be ordered to fit any two-rod arbor. More information can be found at their web site: