A Curious Sign

I've done some more determined Google'ing and this is apparently one of four NCN milepost designs - 'The Cockerel'. See Sustrans

The design seems to be part of a time-themed puzzle 'The Millennium Time Trail' which had the following instructions:

Collect pencil and paper rubbings of the disc as a record of your journey.

When you have collected rubbings of all the discs from a certain set, you can send for a time treasure. With the time treasure you can make a three dimensional version of the rubbings you have collected. This will help you to crack the secret code in the design.

As there are 5 sets of discs, there are 5 time treasures to collect. When you have collected all of them, you can tackle the final treasure and win an exclusive bronze time themed sculpture.

There are at least 2 copies of most time trail symbols in each region and the mileposts have been arranged so that the first 2 sets of designs can often be collected during a single ride near a large town. Of course, you don’t have to complete all the stages – you might simply want to keep the rubbings as a memento of your ride.

The five sets are:

T1 - set of four - "The Seasons"
T2 - set of six - "The Continents"
T3 - set of eight - "The Ages of Man"
T4 - set of twelve - "The Signs of the Zodiac"
T5 - set of twenty - "The Centuries"

So I am guessing that this is part of T4, but I can't track down any evidence of anyone solving it, and any reference seems to have disappeared from the Sustrans' website.