The 6-groove bbl on the early 357 Blackhawk is the rarity.
They used that first in production and most all of the guns were the short bbl editions (4 5/8")
Shortly after they intro'd the longer bbl version of the 357 Blackhawk (6 1/2") Ruger changed the rifling to 8 groove.
So the very early 4 5/8" 357 and a very few of the first production 6 1/2" 357 Blackhawks were assembled with the 6 groove bbls.
Then the 8 groove bbls became the norm.
..and FWIW,,Ruger early on had Marlin make barrels for them as Ruger's mfg'g facilitys were not up to the job at the time.
AFAIK the bbls were .22cal and for the Standard Auto and Single Six series of guns that Ruger made at the time.
When Marlin was making the production move to Microgroove rifling for their .22calRF firearms in '53 or '54, Marlin dropped production of bbls for Ruger.
I don't know if Marlin made any other bbls for Ruger later on, Microgroove or otherwise.
It was not unusual in the industry for one mfg'r to make parts or assemblies for another. Keeps the machines running, people working and $$ changing hands.
Now instead of one mfg'r making parts for another, you see more of the outsourcing of parts mfg to independent companys, smaller shops, ect instead of their competitors.
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