This is one probably of the older-timers.

Having recently purchased a set of Wratten gels, I took the #38 blue out to see what it might do for our clematis. I gave it a factor of 2 just looking by eye, then came back in and tried to find the actual factor. I cannot find one listed. The old Adams Natural Light Photography lists a C5 blue with a factor that "may be" 6 with panchromatic film. Any correspondence there? Yes, I'll see the negs when developed, but I do wonder why no factor is (apparently) given. I don't have a copy of the old Kodak guide to Wratten filters.