Gantry speed limit - Sc7000

Please can someone help me with a query about a speed limit I have seen displayed on electronic boards on the M3. I have twice this weekend seen a 40 limit displayed without a red circle but also without the zmber flashing lights which Highway Code says are present for advisory limits. Is this still advisory because of lack if red circle? I cannot find an example of a limit displayed like this on any official site

Gantry speed limit - Bromptonaut

Unless it was on 'Smart' motorway then (AIUI) it's advisory. Only a legal limit with the red surround. No idea why the 'wig wags' were absent.