A little more digging came up with this image from a 1957 Tyzack catalog
http://trowelcollector.blogspot.com/2014/12/s-tyzack-1957-catalogue-
now-available.html">http://trowelcollector.blogspot.com/2014/12/s-tyzack-1957
-catalogue-now-available.html


with a clearer illustration of a line pin.  More info on line pins here -
https://www.wonkeedonkeetools.co.uk/line-pins/what-is-a-line-pin

Is that the whatsit under discussion?

Phil