Dear Photographers All,

I'm brand-new here and to photography as well. I've a "Miniature" Crown Graphic 2x3 with a Graphex Optar 101mm f4.5 on it, and I am going bananas trying to find a red #25 filter for it --the lens has threads, and evidently the thread size is 31.5mm. I have tons of film to shoot with this camera, but it seems silly to go out there without a filter... Any idea how to resolve this apparently intractable problem?

I don't know about the 101 Optar, but for the 135 Optar the intended filter system uses the slip-on Series adapters. I suspect the intent was the same for the other Optars.

You measure the outside diameter of the lens front element group then use that measurement to determine which Series filter slip on adapter you need, hunt until you find one and the series filters you need.
See http://graflex.org/helpboard/viewtopic.php?t=264 for the series sizes.
Site search "series filter" with the "search all the terms" selected to see all 117 posts about series filters.

The threads you see on the front inside are the front element retainer threads not filter threads.

Some ebay sellers sell the slip on without a retainer ring. The retainer ring is needed to hold the filter in the adapter. Some retainer rings are dual threaded allowing the use of two filter at a time.

For the 101/4.5 Optar you'll need a 4-1/4 in. (31.5 mm) slip-on adapter ring for Series VI. The front of the adapter ring is threaded. You'll also need a Series VI retaining ring. (Series filters have no threads.) You put the filter on the slip-on ring, then screw on the threaded retaining ring to hold the filter in place.
I managed to find this set on the auction site years ago. It takes a little patience, but they're still out there.

Are you sure that is 4 1/4 as that will be a huge lens.
1 1/4 matches the mm measurement given.

I'm a sloppy typist. Of course 1-1/4 in. is correct.