Correct, lugged frames of aluminum and steel thermally " glued " together. From what I understand from past recollection is that it is an Epoxy based thermal set.
I purchased a Raleigh Technium in 87, assembled with this technology. and it was explained to me by the LBS owner.
I have used Structural adhesives to bond Electric Motor frames and the technology has advanced and become common in industrial and commercial applications.
LOCTITE is a major mfg of industrial adhesives.