Hi Lance @lance-comfort,
Unfortunately I am not going to be able to be much of a help here. Mammalian ribs are notoriously difficult to identify down to any precise lower taxonomic level. For example, out of the 400,000+ fossil we have here at the FLMNH none have been positively identified as Castoroides (in fact, only two have been identified as beaver at all, and they are complete ribs). So, I guess what I am trying to say is it might be, but it might not be… sorry I can’t be more helpful!