BTW, I checked the original nuts and bolts and they are 3/8". This is approximately 9.5mm and threaded shank. Not ideal for a 10mm hole through the additional width of the spacer.

Not really ideal fitting 3/8 bolts in a 10mm hole. Once the bolts are done up its actually the friction between the flanges that transmits the load and not the bolts so as long as there are locating steps to centre the components the bolts wont be critical for alignment. If they were using 3/8" they could at least use aviation grade AN fasteners, washers and jet nuts.

The new bolts seem to be holding, not that they are getting much stress in this weather.