[Milsurplus] I-177 Tube Tester Question

[J Forster]

In researching the I-177 Tube Tester, I came across this eBay ad:

"Up for auction are 10 Type 24 tubes of various sorts all have been tested on
my calibrated I-177-B Army tube tester. They
 read as follows where 1500 is the minimum good. 3 Philco(1300 w/ weak
display, 1300 w/ strong display and 1 dead) [snip]"

What is meant by the term 'strong display' or 'weak display'.?  I know little
about the unit, but it appears to read out Gm on a meter.

Puzzled,
-John