GB Scott 357d 2 1/@ Wilding With 6d Surcharge?

The following is a stamp which has a 6D surcharge on its face. I have identified the stamp in Scott is a 357d as it has the graphite lines on it's reverse. What is confusing is I cannot find a listing for the surcharge in either in Scott or SG Simplified. I do not have SG specialised. Could this addition have been made by a previous collector, as a part of the cancellation?

The stamp is not in the best of condition so usually would not retain except for this pesky 6D.

Any help would be appreciated.

But rather, you have a stamp with a portion of a slogan machine cancel on it advertising "recorded delivery" being cheap and effective. It can end up on any stamp of that era:

Slogan cancel

Thank you to you both for providng me a copy of the slogan cancel.