Thank you for your advice, "lysol". I am delighted to find that under the ALT button is a plastic oval which holds the button by means of a lug under the button and the oval seems sprung within a metal cube just smaller than the button. I see no rubber dome.
Since my first posting I have read on a French technical 'museum' online ("obsolete technology", I believe) that only 2000 of these models were made due to uncompetitive pricing, so they are rare and aesthetically remarkable. Thus if selling it on I would be concerned that it not be broken for parts, albeit it is not of functional value.
Thank you again for your interest and prompt reply.
Best regards
S D