Mick nailed it…  

here’s the reference
http://www.buildingconservation.com/articles/toolsoftrade/toolsoftrade.
htm">http://www.buildingconservation.com/articles/toolsoftrade/toolsoftrade.htm<
/a> <http://www.buildingconservation.com/articles/toolsoftrade/toolsoft
rade.htm">http://www.buildingconservation.com/articles/toolsoftrade/toolsoftrade
.htm>

and you’ll find a fascinating treatise on the use of them, illustrations of
other versions, and why shoudl use one.

Yet another (tm) instance of an object being worthless in one auction, targeted
to bookbinders, and valuable in another - targeted to building conservation.
There is a huge upswing in the idea that lime mortar is a Good Thing - better
for the environment, less energy intensive to produce, kinder to the building,
and so forth.

Richard Wilson
Yorkshireman - with a south facing stone wall needing pointing because the
clowns who built it used cement, and the cracks around each stone now allow the
weather in.