I contacted Theo Rehak at Dale Guild Type Foundry, who use the term in describing their fonts. Following is his response.
George,
We use ATF packaging with reglets of tied lines of type. Our fonted "lines"
of the type measure 36 picas (or 6") in length. The "line font" term is
simply the count of the number of these lines in the package. Modern type
artisans often figure the weight of type they are buying by multiplying the
number of lines in the package by a fixed known value. For instance, a 6"
line of 16pt foundry type weighs @ .333 of a pound: so..... 6 lines of 16pt
foundry type weighs 2 lbs. etc.
Theo Rehak / DGTF
I found examples of Dale Guild Foundry Type faces on the following site
http://order.nagraph.com/page11.html