Well done Big 240; welcome to classic car ownership. This is just normal business for 40 year old cars - and it becomes even more prevalent with 60 year old motors.
Most times when you take your motor car apart to fix or clean something you will find a fastener missing or loose, a cable wrongly routed, a split tube or a bare electrical conductor: it is one of the things that makes older vehicle ownership so interesting. In general never throw bits of your vehicle away (when you find, fix or replace them), keep them all in a spares box for when something similar falls off.
PS. If you didn't find the missing bolt in the footwell then the previous owner probably threw it away after it fell out - my point exactly.