Sorry to be late to the party, but I have been traveling and just got back to Iowa yesterday.
I have two similar holders, which I purchased off e-bay. I paid $99 for one of them, and got the other for about half that. Sorry, I don't remember the exact price. The holders I have are different, however. Mine are labeled "Bromoil" holders. They are the same thickness as a modern holder. Unmodified, they were about 1/4 inch wider than a standard holder. I simply placed my regular holder over the Bromoil holder and marked the size with a pencil. I then trimmed down one side with a table saw. My holders have a hinge on one side which prohibits trimming both sides of the holder. There is no ridge along the top, but I have not had any problems with light leaks.
The holder you show looks older than the one I have. It also looks a bit thicker. Be sure to check the thickness of the holder (and film/plate position) before trimming down the sides.
The insides of the holders look the same. I clean the insides each time I use the holder for wet plates. I wipe down the inside before the plate goes in and after taking the plate out for processing. Be sure to clean the back of the holder.
I really like my holders. I can have one holder and it will take many sizes of plates. I hope you can make this one work for you.