Further investigations:
(a) removed all fields from 'Set Displayed fields'. This result in a blank header apart from the two periods
(b) Compared the .layout & .mbv files from the two systems - found no 0-length fields
So I made a new tab, applied the filter for the Popular tab and the two periods had disappeared. So I saved the panel layout and applied it to the problem tab and the two periods disappeared. I then re-applied the saved panel layout from the misbehaving tab and the two periods reappeared. I compared the 2 .layout files and they were identical!