Fitting a post-'62 seat...

[e3steve]

...to a '61 frame.

This image is, in particular, for Isiah (56_oval). If you want a detailed drawing it'll take a little longer for me to measure and draw the extension bracket.

I drilled the frame's flattened-out tubes, just ahead of the original slot, and secured the alu brackets, each with a s/s M8 'button head' Allen with star (shakeproof) washer and dome (acorn) nut.

There's a small alu packing piece -- not shown here -- 30mm square (from the same strip of 4mm alu) that has an 8mm hemi-notch filed out of one edge; it slots in place around its seat bolt (on top of the modified part) once the seat is on, then clamps in the gap when the seat is tightened. This prevents the seat brackets from deforming under the strain of the bolts' tightening.