Mini seats.

Remember these?

I had a pair of them back then. I had Mini seats in the Mark 5, albeit with the superfluous bits chopped off the bottom and the seats bolted directly to the sheet metal floor that Mark 5s have. The Taylorspeed covers really transformed them into buckets and the side support worked really well too.
From the same Wheelspin, July 1967 came this snippet from the gossip column.

If it's the one I'm thinking of, it wasn't just a guide post but the rather solid sides of the Belair railway crossing, made out of railway line. Not absolutely certain, but Peter Mounsey rings a bell. It wasn't Rob Butcher, he did his on a trackside Stobie pole at Mallala ( same one that claimed Scott Baxendale and Richard Gac I think). Anyway, both Mark 7s lived to fight another day. Whoever heard of a trackside Stobie pole anyway?
And from the classified page......

A smart looking TF for $375, they were the days. I bought my TC about then - for $500. Must have paid through the nose. And was that the Capricornia/Ricardian Clem was selling for POA?