Blade removal

I purchased a Gravely ZTX 52 last year. I am trying to remove the blades to sharpen them but cannot get them off. A person at the parts counter where I bought the mower told me that it was because the spindles are reverse threaded. I do not believe that is true. I read online that the blades come from the factory so tight that I will need an impact driver to remove the bolts. The service department at the dealer couldn't tell me what size socket I will need. They said either 1 3/8 or 1 7/16. I bought both but they are both too big. Can anyone tell me what size socket I need to remove the blades?

Blade removal