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Add EXTENSION attribute value (ex: EXTENSION - PART, set number like 123_PART.prt) as suffix to the number of the object using OIR.
extension is a custom attribute created under wtpart.
This article has the steps for your requirement
Do you know it the sufix is placed before.".prt" as @SS_10547676 needs or it is places in the end of the ".prt" so by the article it could looks like "123.prt_PART" ?