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There is no way to install a hold open on the Model 552. It is a strange design and it will not work. Adding any weight to the bolt will slow it down and affect the operation of the gun. I have a thin piece of poplar (about 1/8") cut in the shape of a "T". The top is the length of the opening when the bolt is pull to the rear. The other part is used for a handle to put it in place and remove it.
The manual has complete disassembly instructions for a thorough cleaning and lubrication. You can download or print a copy at the following web site.
I hope this helps you,