|A basic socket lug with no ornamentation on a cheaper frame. Plenty strong, but heavy.|
Looking at the lugs used by Sekine over the years, we see several different types. Obviously, you'll find the plain-cut socket joints used on bottom-end frames; but there's variance through the years and more expensive models. Many of the lugs resemble, at a glance, the famous Nervex lugs from France.
|The famous "Continental-cut" lug appeared on thousands of Sekine's mid-70s SHT, SHS, & SHL frames. The lug features fancier cutouts, and retains its strength while reducing weight and adding aesthetic appeal.|
|Another basic lug, yet it has some ornamentation|
Sekine probably had a few suppliers for their frame materials, (for example, Shimano dropouts) but most notably Tange Industries supplied many of the lugs, and much of the steel tubing used to build the frames. Tange also made the forks for most of the Sekine bicycles.
The lugs pictured above are on frames made perhaps a year apart, around 1980. The one on the left is a cheaper frame; the lug is chunky and heavy. The frame on the right has nicer lugs, with long smooth tangs and more defined curves. I'm not sure what Sekine used for brazing material, but I'll find out soon.