I have a Shimano RD-TY20 derailleur with a date code of 1989. It is mounted on a frame which is marked as 1984. This frame is made of steel, and the derailleur hanger is part of the frame.
I want to modernize the drivetrain (Please reserve your judgement on whether this makes sense: It does not.), but I'm having doubts about whether I will be able to mount more modern RDs. Here is an exploded view of RD-TY20 showing how the RD is attached to the frame (see elements 18 and 19 under "Direct Attachment Type"):
A bolt is stuck through the RD, which is then attached to the frame's hanger with a nut. The bolt is narrower than the M10 bolts built into modern RDs, but the nut that secures it to the frame seems to be about the same width.
What is this kind of attachment style called? Where can I get more modern parts with this attachment style, if at all?
Googling for "Direct Attachment derailleur" returns some relatively recent innovation and is of no help.
Of note, perhaps, is that the integrated hanger on this frame does seem to have some threading in the hole, but I don't have any way to check if it would accept an M10 bolt.