When you say 2-70 through 4-71 marked dates, does this mean marked on the clasp like eg 1039, or marked on the hinges like the ones in this thread? I'm curious if the ones we say unmarked also have the small stamps on the hinge or if there are no marks on both the clasp or the hinge.
Both my 4-69 and 10-71 have the no. 12 unmarked clasp.
I also noted but didn’t mention a difference in the sides on the links. My 69 has brushed sides with a sharp edge. The 71 sides are polished and slightly rounder. It is possible that someone polished them but they are very even and consistent, enough so to make me think it left the factory this way. (Both of these have the Omega no. 12 clasp, although the 71 came from Japanand the 69 from the USA.)
Like you mentioned, given the long production, there seems to be differences.
(The 4-69 came with my 145.022-68 from the original owner. The watch has a DNN close to 70 bezel, so parts like the bracelet were likely changed. I don't suspect the clasp was changed though.)
I suspect people would take a 1039 over the 1175, but for the price (and possibly extra sturdiness), the 1175 is a great choice. Unfortunately....like everything vintage speedy the price may change.