Glad to see you got it in the car!
I came across the same problem with the filters. I just ignore the scales on this and other colour heads. I simply increase magenta to add contrast or yellow to decrease contrast. This does mean that the exposures will change with the contrast, but I tend to find that even with the ilford mg filters.
I also found the 504 prints flat, and on old negs that print fine on grade 2 I have to have the magenta up to around 60 to get a comparable print.
My 504 is now heavily adapted When the transformer and timer packed up I couldnt source another cheaply, so it is now rewired with a different light source.