The Royal Guardsmen.
Interesting history here: https://www.crazedfanboy.com/spotlight/guardsmen.html
Which includes this picture:
Which should probably give an approximate year of birth for Burdett if you have everyone else's - this does seem to relate to a reunion sometime around 1977/78, so possibly year of birth 1949/50?
When Burdett quit in 1968, he seems to have moved to Bucks County, Pennsylvania according to the article. The official site for the band which is stated in the article to be maintained by Burdett no longer exists, which could be a worrying sign?
This article suggests Burdett might have been originally from Minnesota: https://www.tampabay.com/things-to-do/music/Royal-Guardsmen-return-Snoopy-in-tow-50-years-after-Florida-s-first-pop-hit_167682029/ and this linkedin profile suggests he is in Saginaw, Michigan: https://www.linkedin.com/in/john-burdett-a9896a20
Apologies if you've looked at all of these.