The rear dropouts appear to be shorter than the 3755/56 model. I'm aware of two shorter Simplex dropouts with eyelets and the hole at the back of the axle flat. In 1977 they introduced the 4130/31. Then, sometimes during 1978-1981, they introduced the 4264/65 which was even shorter and had a stop and threading for non-Simplex derailleurs. The subject bicycle looks like it has the 4130/31 but I'm not seeing any adapter on New 105 rear derailleur. Regardless, the dropouts would appear to push the date forward to at least no older than 1977. However, given the recessed brake fittings, I doubt it's very late 1970s. I'd place it into the early 1980s and be leaning closer to mid-1980s based on what appears to be a plastic cable guide under the BB shell and the fade paint. That makes me wonder if the 1st generation 105 crankset is OEM and the New 105 was an upgrade for the SIS derailleurs and SLR brakes.

Given the metric tubing and mid-range frame features, my prime candidate for tubing would be plain gauge Durifort/Vitus 888. Typically, in a metric tubeset, this tubeset would use a 26.2mm seat post. This is only one size off the current post and could have been used is the tubes were slightly distorted during frame manufacture. Another less likely possibility would be the metric version of plain gauge Columbus Zeta, which would use a 26.0mm post.

I am a bit surprised that there is no serial number, as the workmanship is typical of a production frame. Maybe there is something under the cable guide?