Like yours, my 504 doesn't have a Red Filter or even the Metal Post that Steve describes.
I very quickly got used to not having one
However, I use Ilford MG Under the Lens Filters – and it turns out that there is a red safe light filter included
Having been weaned of the Red Filter, I have never gone back to using it again, now its available
I prefer the separate filters to fiddling with the CMY on the head, as I can slide the filters in and out of their carriage without disturbing the head
Martin