Apologies, have been absent from the forum for a while - have to admit to having my head turned by a couple of Ducatis and a couple of Hinkley Triumphs and the Hondas have been somewhat neglected as a result!!
Clearly the world is a small place - I lived in Mexico until 2012 and shipped back to the UK two 40' containers, full of bikes and a '65 VW Split Screen combi that I accumulated during my stay. Most of the bikes were Triumphs from the 1960's, since there was a Triumph agency in Mexico at that time. I did, however, have the good fortune to buy an ultra low mileage K6 and a Sandcast. Mine is not as low a number as yours, but was purchased by the original owner and imported into Mexico from new in 1969, so clearly there were, at one time, at least two Sandcasts in Mexico. If yours was originally purchased by the son of the Mexican president, at that time, he would have been Gustavo Diaz Ordaz, right?
Would love to see some photos of the bike as you found it and as it is now!!
Cheers,
Richard