Mr. Hallquist:
See a lot of Fritz Huels' examples do you? He appears to have been a Gatlin-gun mechanic and was active as early as 1873. The engraving concern Marr(John) & Richards(George L.) was founded in 1855 by John Marr and look to have been one of the best of their time. Are you certain the tubes aren't pattern welded? The top rib looks to have some pattern and I would suspect he used some muzzle-loading tubes? Lovely eyes on the hammers.

Kind Regards,

Raimey
rse