Update: About 3 months ago I had occasion to talk to an employee of the company that processed the negatives that came in from two different jobs in California and Florida. Someone set a temperature control on the scanning equipment too high and condensation formed on the lens during the scan of the aerial photos, producing the circular droplets. Being a small part of the overall image, they were not noticed until some eagle-eye mapping enthusiasts noticed them and posted them in several different bulletin boards - including this one.