[Lynn Eades]

I dont know what it means, but the 6-71 as used in the Mk4 and mk5 Valentine is/was refferred to as a "6-71 low head". (at least here in N.Z.)
For those that are interested and dont already know, Detroit Diesels have been built in many configurations. the first digit refers to the number of cylinders, and the following 2 digits refer to the cubic capacity of the cylinder in cubic inches.
They were built in 3 sizes (that I know of) 53cu.in., 71cu.in., or 92 cu.in. So the 6 -71 was a 6 cylinder (in inline motor with a swept volume of 71 cubic inches per cylinder. In the case of the "V" motors, the "v" followed the number of cylinders. So for example, an engine could be an "8V92" or a "6V53".