Hospital steward jacket question

[Billy1977]

Hello everybody, I was wondering if someone could tell me if this hospital steward's shell jacket seen here was commonly worn for everyday wear in lieu of the frock coat? Or is this some one-off item that was specially made for some individual hospital steward and not common? (Note that it appears to have cavalry buttons for some reason).

 

[Billy1977]

UPDATE: I just found in the Time-Life Echoes of Glory book for the Union a very similar shell jacket for a hospital steward but it lacked the green collar, instead having a plain (and shorter) collar that matched the jacket.

 

[Billy1977]

UPDATE: I found this photo that seems to corroborate the concept of hospital stewards wearing shell jackets.