I use some 52 mm color B&W filters for my Classic Nikons, Red, Yellow and Orange. Of course they cannot be placed on the lens, but can you just hold them close, over the lens as you take your shot?

Is there a holder that you can purchase or make to hold Graduated Cokin P series Neutral density filters?
Is this even bothered with, .. as the camera may well predate such filters and one should just shot it 50's style?

My two cable releases will not screw in properly to the Graphic. To fat at the tread end, although I believe the tread size is the same as my modern cable releases, I wondered if this is a specialty item for the Graphic?
Either way, does anyone have a source and manufacture # so I can order one so it will fit?

(Note: It certainly different shooting large format over 35mm manual classics. It makes them feel like a point and shoot camera.
Haven't shot many yet, but twice I took a shot, having to laugh, because I hadn't pulled out the dark slide. One guy asked, 'What's going on?' I answered.. 'Good question!" )

I can't help you with the filters.

My three cable releases all purchased in the early '80s fit my Graflex and Kodak shutters. I take it that you meant lens shutter not a body shutter release on a Pacemaker Graphic?

As someone here once said "Great images are recorded on dark slides."
I use blue painter's tape and a Sharpie for notes on the back of the camera. e.g., film speed, pull dark slide, etc. and sometimes I still forget to pull the dark slide. Not always a bad thing since it allows you to have a new thought about the photo you want to take.

Welcome to the club and have fun along with the rest of us.

Previous posts of relevant interest:
http://graflex.org/helpboard/viewtopic.php?t=264&highlight=series+filter
http://graflex.org/helpboard/viewtopic.php?t=1727&highlight=series+filter

Measure the outside diameter of the lens barrel, find a slip on adapter of that size, get a step up ring that will screw into the slip on adapter and adapt it to the Cokin P holder.

Current cable releases should fit your lens. Check the threads on both the cables and lens for damage, or the lens socket threads may be dirty and clogged.

What 45PSS said (re: appropriate-size slip rings for your lenses), and to repeat what's been posted before (since I posted some of them!): the series VI thread size (i.e., the threads on the outer end of the slip ring) is 44mm. Google up something like "44-52mm step-up ring" to find availability; the idea is that your 52mm b/w contrast filters will screw into the step ring which will screw into the slip ring, and you're good to go.

There are a couple of different thread configurations for cable releases. One is the standard tapered thread used by most cameras, including your Graphic. Then, there is the one that has less tapered, or not tapered threads. These are rather rare and were only used by a couple of manufacturers. The first thing I would do is check to see if a tip has broken off on the socket. I've had that happen more than once. If so, a tiny easy-out tool will get it out, or a file tang ground down until it will fit into the broken piece. It will screw right out. For the filters, the suggestion about a 44mm to 52mm adapter ring is a good one. The proper Series VI slip on adapter and that one puts you in business. Much easier than trying to find a full compement of Series VI filters, hoods, etc. 44mm adpters are not common, though, and usually are 44 to 49mm. You can hold your filters against the front of the lens. It works as long as no light gets between them, or is shining on the reverse side. Duct tape is your friend.