The message
from "Franz Heymann" contains these words:

What's a frog in this context?
The indentation moulded into one face of a brick is known as the frog. I
have no idea why.

Probably from a similar-shaped indentation under a horse's hoof.

--
Rusty Hinge http://www.users.zetnet.co.uk/hi-fi/tqt.htm

Dark thoughts about the Wumpus concerto played with piano,
iron bar and two sledge hammers. (Wumpus, 15/11/03)