In fairness to them, they probably get a lot of parts bought like that, then the customer wants a refund when it doesn't fit, even though it looks the same.
I know you said you would assume liability, but with the ability to get visa/PayPal to reverse the transaction they've probably been screwed before and have decided it wasn't in their financial interest.
It's not like they decided that for no reason at all
Now, if you really wanted it, I'm sure you could email them from a different address and say you have a PMxxx lathe you purchased from some guy that was missing xxxx parts.
Note: I am not a PM customer, never have been, likely never will be, and have no dog in the fight, nor would this sway my decision whether or not to purchase from them in the future, but just simply always two different views to the picture.