The issue i found is that 'OtherIdentifyingInfo' is a collection of objects (i suppose)... so you need to use -ExpandProperty on a select statement, like this:

$helper = $_.hardware.systeminfo | Select -ExpandProperty OtherIdentifyingInfo

$helper variable will get all the objects associated which OtherIdentifyingInfo, so you need a foreach to find the attribute needed. I think you should find what you need on 'IdentifierValue' and 'IdentifierType.Label' Attributes

see if this helps