Old lenses do have a certain charm... and many quirks. This is often the case for older American made lenses like this Graflex Optar 135. While the lens still works just like new, it does lack a feature that we now take for granted: a filter thread. While there is a thread on the lens, it isn't metric like all filters sold. It's sole purpose was to allow the front retaining ring to be screwed-in.

So, Kodak produced adapters for such lenses. They're essentially press-fit aluminum rings with a retaining nut on the front. They take un-threaded filters that just slip-in and that get held by the lock nut. While this is all nice and well, it does offer a few problems to the modern photographer.

First, filters for the device are somewhat hard to get.
Second, their usage is not the easiest as you have to hold the lens tilting up to insert a filter without running the risk of dropping the parts. This gets old very rapidly as you loose the composition you're working on every time you change filters.
Third, you have to hold three parts to change between two filters.

So, I got tired of all this and decided to build an adapter. At first, I thought I'd use the thread already present on the Kodak adapter. Surprise! It's in threads per inch (not compatible with modern parts). So I had to look in another direction.

I also had to order parts almost one by one until I managed to get something that works well.
But now, I save you the trouble of having to figure it out. The principle I use can be adapted to other size lenses. Only the dimensions will change.

Step 1: Required Parts and Tools

Here are the parts you will need to be able to use regular filters on the Graflex Optar 135.

Kodak Series VI adapter ring 1½ in.-38mm (with its lock nut)
42mm-39mm filter step-down ring
39mm-52mm filter step-up ring (actually, it's 39mm to whatever size filters you have)
39mm UV filter

And the following tools:

Sandpaper (fine grit)
Dremel tool with the cut-off disk (or a metal hacksaw if you don't have anything else)

Step 2: Adjusting the Size of the Step-down Ring

First, you take the step-down ring and you use the sandpaper to bring the outside diameter to a size that will allow the ring to fit inside the lock-nut's collar. You don't need to take off a lot of material so it's easier to do it by hand.

I took the ring in my hand and pulled it against the grain of the sandpaper while forcing the ring to do 1/4 turn. Repeat for the next quarter turn and keep on doing so and checking the fit from time to time. It took me maybe five minutes to bring the diameter down to the proper size.

Step 3: Cut Slots in the Ring

Next step is to cut shallow slots in the step-down filter ring that we just adjusted to size. You don't need to cut more than 1 thread deep. This is not mandatory but could come in handy if you ever need to remove the filter. They allow you to use either a spanner wrench of simply a screwdriver to get the filter loose if it should ever get solidly locked by dust and grime (and yes, it does happen).

Step 4: Putting Everything Together

Now, you just need to put the step-down ring in the lock nut (with the notches towards the lens) and attach the filter. This is the point where you can add the step-up ring so that you can fit your favorite filters.
Screw the lock nut to the Kodak Filter Adapter and press it on the lens. You're now ready to use filters on your Graflex Optar.

I hope you enjoyed this simple solution.

Should you be stuck with a filter adapter that is missing the lock nut, you can always use epoxy to hold the step-down ring. It won't be pretty or collectable, but it will work.

For different cameras with differently sized lenses, a host of companies produced Series filter adapters which come in various diameters. Here are some of the various Series filter diameters. I had a hard time finding them so there might be some errors in the following list.

Series IV => 20.6 mm
Series V => 30.2 mm
Series VI => 41.3 mm
Series VII => 50.8 mm
Series VIII => 63.5mm
Series IX => 82.6 mm

Filter adapters written in Arabic numbers are often of a different size than those written in Roman numerals. Verify everything closely.

Wishing you Happy photography!