A couple of things.
First, to the OP: A Wratten 21 orange filter is going to be hard to find. You may be better off with a #23 light red filter; much the same effect, but just a bit stronger, and much more easily available. The next choice would be to go down to the #15 orange - less strong but more effect than the #12 yellow filter. The cuts on the three filters are as follows: #15 520mn+, #21 540nm+, #23 570nm+. They have rather sharp curves, rising from 0% to almost 100% transmission within 50-60nm.
A conversion filter like the 85s have a gradually rising curve that starts at about 385nm but still passes about half of the green light (transmission is 50% at 550nm, whereas a 21 or 23 filter is going to cut almost all of this wavelength out). However, it might be worth a try if you have one lying around. You'll have to test yourself to see if the results are what you need.
B+W filters 040 and 041 equate to Wratten #16 and #22 respectively. You might be able to find one of them running around...
As for Series VI... I converted all my Series VI to more modern mm sizes with step-up rings. You may be able to more easily find a larger filter in a color you can use and use it on your Series VI adapter with a step-up ring.
Now to "Reciprocity Failure." Actually, this is correct. The "Reciprocity Effect," or better, "Reciprocity Law" is the characteristic of film to render a certain amount of density for any reciprocally equal amount of exposure (expressed as "time x intensity"). I.e., 1/4 second at f/32 = 1/8 second at f/22 = 1/15 sec. at f/ 16 and so on. The exposure factors of time and intensity are reciprocal; as one increases, the other decreases proportionally. Film reacts this way over a wide exposure range. The failure of this effect at longer and shorter times is the "Failure of the Reciprocity Law," or, to be more concise, "Reciprocity (Law) Failure."
And, with all due respect to Gerald, the term "contrast filters" for black-and-white photography has long been in use, by Kodak, in the Wratten literature, etc., etc. They change the "relative contrast" of the rendering of different colors by blocking some more than others. Mostly, the term was used to differentiate filters used in general photography to change the distribution of tones in a scene from more specialized filters like filters for making color separation negatives, etc. That's fine with me.
Best,
Doremus