So I took a look at my filter.
There are no numbers on it anywhere. There is no sticker on it anywhere.
Outside of the frame has some arrows for airflow direction, and some words, but nothing to identify a part number for it.
It also doesn't show up in the Red Dot parts catalog under filters (but the R-2400 evaporator isn't there either)
Dimensions are 17-5/8"x5-3/8"x1"
(doing a search with those dimensions doesn't turn up anything for me)
There is always the chance if you send Red Dot a message they might be able/willing to help you. I think I remember hearing good things about their customer service for civilians with military surplus stuff.
Red Dot designs and builds mobile HVAC systems and components for a wide range of heavy-duty commercial and military vehicles
www.reddotcorp.com