Careful there. Check FRN 23400.
My understanding on this is that the FRT for the Mk III service you brought up are made by Webley and Scott, rather than Webley (Webley and Son).
Allegedly, if it has the Webley and Son HOLLOW base winged bullet logo it's Antique. If it doesn't (ie has the SOLID winged bullet logo) it means it's made by Webley and Scott and not Antique. Thats my understanding, and as Polkey mentioned above, they have approved and lettered guns in the 78000 range as Antique, even going as far as to de-register it. The example he's talking about had the HOLLOW base winged bullet logo. I have also heard what 55Recce mentioned about the serial number location being a deciding factor, in that only civilian guns are antique. The serial number location on the lettered example in the 78000 range has the serial number on the side, as well as the HOLLOW based winged bullet logo. The idea that having the serial number on the side makes it a civilian gun makes sense since Polkey found the sales ledger for it from Army and Navy CSL (which the gun is also marked on the top barrel flat). I have a Mk III in the 600 serial number range that is also marked with the HOLLOW based winged bullet logo, and the serial on the side, with P Webley and Son marked on the top of the barrel, lettered as Antique by the RCMP.
So in summary, what we do know is this;
Sn 6xx is marked on the left side with the HOLLOW based winged bullet logo (Suggesting Webley Manufacture, vs Webley and Scott), with "Webley Patents" marked beside it, with no model designation on the frame. Sn is marked on the right side of the frame, making it Civilian retailed. Also Marked "P Webley and Son" on the top barrel flat, confirming the manufacturer as well as further proving it was Civilian retailed. "Mk III" stamped on left side top strap above cylinder. This example has been Certified and lettered as Antique by the RCMP
Sn 78### is marked on the left side with the HOLLOW based winged bullet logo(Suggesting Webley Manufacture, vs Webley and Scott), with "Webley Patents" marked beside it, with no model designation on the frame, same as Sn 6xx. Sn is marked on the right side of the frame, making it Civilian retailed, same as Sn 6xx. Marked "Army & Navy CSL" on the top barrel flat, further proving it was Civilian retailed. "Mk III" stamped on left side top strap above cylinder. This example has been also been Certified and lettered as Antique by the RCMP
Another point to note, on both FRTs, it lists the lowest knows serial number for the Mk III is 101, and the highest being 80012.
Though having said all that, 2 of the 4 entries on the FRT for Webley and Scott are for Antique with the other two being Prohibited and Restricted. I guess as usual all this is a mess and nobody, including the CFP really knows.
I have attached a screen shot of both so we can compare the manufacturer between the two.