I need to add a secndary U320EMM to my PowerVault 220S.  Dell parts said this is a low stock item.  Tow refurbs are available on eBay.  Could you tell me the difference between the two as the eBay vendo is offering both:

Dell PV220S U320 EMM (YD893)

Dell PV220S U320 EMM (Y1987)

Thank you