It's a Poser fault. You can't save more than one smart prop in a single prop file. AFAIK the only way to do it is to hack the pp2 file, which is what I do. In this case, the best approach for you would be to export the bar and the two plates as a single obj file, then import them again so they become a single object. Then you'll be able to set it up as a smart prop the normal way.

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