@ScruffR - Exactly what I was looking for. Thanks!
I found the following link searching the Particle Community for “extern”, and will reference it here just in case someone else has the same question: http://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files-in-c, referenced from Extra code files
P.S. That shows how long ago I was programming in MS VC++…