The dropout was originally held in 'position' by two m4 (?) button head socket screws . I suspect that the aluminium dropout
was also glued/bonded to the magnesium alloy ( like other parts of the frame ) and the screws were only used to
hold the dropout in the correct position while the adhesive cured . The screws being too small to contribute much
to the strength of the joint
To reattach , you would need to completely clean the area where the dropout fits in the frame removing any trace of adhesive etc .
going back to bare metal . Check that the screws ( if you have them ? ) will fit correctly in the frame , buy a suitable 2pk resin
adhesive , clean everything again in isopropyl alchohol and immediately assemble the parts removing surplus adhesive before it sets
Hope this helps