Why do some casebacks on vintage speedys have the swirly marks on this inside. The 69 speedy i own does not have the swirlys. Any clue? Most casebacks i have seen have these swirlys Mine is just plain.
I have read on this forum that there were three different caseback possibilities. I'm sure someone will be along shortly with some photos for you as well. Yours looks just fine to me.
Those swirlys are marks left by a tool with a small round "tip" that polished the inside of the case back. But I don't remember what was the reason for it. I know on some movements (not omegas I think) they used to do this for added visual effect.
It is called Pearlage and is a way of demonstrating the art form of a watch. IIRC, it is done using a rose engine. Some watches even have pearlage on the dial side of the main plate where nobody except for a watchmaker will ever see it, so it can just be a case of having particular pride in your work.
Perlage is the name of this decoration, and it's quite simple to do. In some fields it's also referred to an engine turning, but to me that never made much sense. Perlage is a series of overlapping round swirls made in the material - here is a photo I took at one of the Perlage stations in the Patek Philiippe factory just outside Geneva: It can be done in different ways, but it generally involves a rotating spindle, and these days is done primarily using Cratex tipped round bits. Cratex is a rubberized abrasive, but I have done Perlage myself just using a drill press, a piece of wooden dowel, and some abrasive paste... As noted the dial side of a watch (where no one but watchmaker's typically look) can have this type of decoration, like on this Panerai 112: But then again the train side of the movement it seems they only bothered to add Perlage between the bridges, and didn't bother doing it where the main plate was covered: Decoration like Perlage or Côtes de Genève is these days seen as primarily just decorative, but if you believe some sources, it was developed as a means of helping to trap dirt that might enter a watch case and keep it from migrating to a location where it might cause a problem. On the inside of a case back that already has an inner cover over the movement, it's just decoration. Hope this helps. Cheers, Al