You can't directly compare CFM without taking SP into account. Typically a high SP requires more current than a similar bump in CFM. If your replacements are actually drawing about half the current of the originals, than you are moving less air. The reason for the different part number, is that many times the OEM will put whatever part number the customer wants. But I rather doubt this is a custom fan, more likely a standard model renumbered for the welder factory.