This is a question inspired by the fine early modified rifle recently offered for sale both here and on other milsurp sites. What caught my eye is the barrel bluing near the early rear fixed sight base which is strapped as opposed to a solid sleeve, as shown below.
Some of the barrel appears in the white, in a edge that follows the contour of the fixed sight base. I would think that immersion in the bluing tank would not result in such a contour. Could this be a case where grease or similar was applied to mount the base, and then only wiped off where easily accessible? My understanding is that sight bases were mounted on barrels while all part were in the white, and blued afterwards. I've noticed a similar contour on other photos of strapped rear fixed sight bases, but this is the best quality photo for illustrating the point.