55 to 66 all look really similar. You might want to just contact Matt and ask him. https://www.hayesdetailshop.ca/our-story
Looks like a '55.2-59 Chevy stepside rear bumper. There was a comment made about a later one. Fleetsides were straight across (looking from the top) whereas the '55.2-59's were curved. Not sure if this bumper style was used up until '66 otherwise bed was the same.