Som filters fade with time.
Ambient light levels and heat can be the biggest issues.
Papers do loose contrast (even MG ones)
I have a set of the earliest of the current (not 40's) Ilford MG filters (late 70's I guess) , and they were not speed matched at all; there was a little dial calculator to aid in swapping filters. I cannot recall which ones fit where. I keep them as a historical item, and have never actually used them.
I think it was the II filters that were suppose to be speed matched for 0-3.5, and double exposure of 4 and 4.5, which is not what your current expereinces with your set seems to exhibit.
The most recent Ilford filters had a 00 lowest setting. The most recent Kodak's were branded Polymax, and went from -1 to 7 I think. I just mailed a set off to a guy in the UK, and did not look all to carefully at them.
I still have a few sets of NIB spares in my stash of too much gear for one guy to own.... PM me if you are interested in a set for $10 and the cost of postage. Let me know if you need the below the lens or above, and the size needed and I will see if I have what you may need.