It looks like the upper left wheel was replaced and they used a square headed (on the inside) bolt to secure it. The shown piece is what they used to secure the bolt, as a cotter pin is holding things in place.
It looks like the pin could be removed and the unit taken apart if you wish. Hopefully this information is helpful and the pictures help with the explanation.
Great vintage barn fresh trolley. Ready to be refurbished to your liking. Refer to pictures for overall condition.Thank you for your interest and have a great day. The item "Vintage Porter Hay Trolley Pulley Cast Iron Farm Barn Tool Rustic 42 & 43" is in sale since Thursday, December 26, 2019.
This item is in the category "Collectibles\Tools, Hardware & Locks\Tools\Pulleys, Block & Tackle". The seller is "simbacurt" and is located in Mifflinburg, Pennsylvania.This item can be shipped worldwide.