I found 3 of these SERIES 1 mystery figures these at a local KB for $3 bucks each. I figured "ah why the hell not? They're mystery figures!" so I picked them all up.
But I don't know who I have! (well sorta)
After some searching, I found some answers...

"On all of the Marvel Figure Factory figures, If you look in the lower left corner of the back of the card, you should be able to see a letter stamped/embossed into the cardboard. The letter is not printed, so you may have to wiggle the card around until the light catches the embossing just right."
http://actionfigures.about.com/b/a/234971.htm

Each of these letters represent a mystery figure.

One box I have is "marked" (actually, imprinted) in the lower left corner with an "M".

According to About.com & Toy News International, (http://adctalk.com/index.php?showtopic=1996063)
This figure is M = Storm (Mohawk, black).

Okay good.

Now the other two I have are both marked in the lower left corner with an "X".

Apparently, Toybiz also used the remaining letters of the alphabet (anything after "M").

Unfortunately, I don't have any info on and letters after "M" other than the letter "T" which is SERIES 1 Captain America.

Now assuming in theory, if I apply Cap being "T" to TNI's list, then we skip the letter "N" resulting in something like this:

A = Unmasked Spidey or "O"
B = Doc Ock with Lab Coat or "P"
C = Grey Hulk or "Q"
D = Unmasked Wolvie or "R"
E = Unmasked Venom or "S"
F = Captain America or "T"
G = Red Daredevil or "U"
H = Yellow Daredevil or "V"
I = Human Torch (Reg) or "W"
K = Unmasked Human Torch or "X"
L = Storm (White) or "Y"
M = Storm (Mohawk, black) or "Z"
*"N" is skipped.*

Now if this assumption is true then my 2 letter "X's" should be:
K = Unmasked Human Torch or "X"

However, I'm only guessing here.

So if anyone knows what other letters (after "M") these figures are represented under then that will be of great help to me.

And just for the record, I do not plan on opening up these figures until I am absolutely sure of who is inside the package.

 

I went through this a few months back. Basically if it's past the rubric, you're boned. Kind of a wild card. I had an N and it turned out to be Storm with White Hair.

I have a revised code listing somewhere, I'll try to find it. I added my own findings.

 

Bizarre...I found a ton of mystery figs. back in the day and I was able to ID them all by the code on the back. Crazy! My advice? Open them!!! Even opened mystery figs. can fetch an okay amount (better than $3 at least) if that's why you are wondering.

 

I found a master list (complete with explanations!):

Series 1 Mystery (12 figures + 3 new = 15 figures total):
1. Grey Hulk
2. Yellow DareDevil
3. Unmasked Vemon
4. Unmasked Spiderman
5. Storm (White)
6. Storm (Black) With Mohawk
7. Doc Ock with lab coat
8. Human Torch
9. Red Daredevil
10. Human Torch with flame off
11. Captain America
12. Unmasked Wolverine
*new* 13. Storm (Mohawk/Silver with gold trim costume)
*new* 14. Storm (Black, full hair)
*new* 15. Variant Costumed Red Daredevil

List of Mystery Codes for Series 1 (12 figures + 3 new = 15 figures total): (Note: Letter Ingraved in back bottom left, Under Hulk Pic.)
*BOLDED “letter” = re-release of Series 1 mystery figures with new designations.*
A or P = Unmasked Spidey
B or M = Doc Ock with Lab Coat
C or R = Grey Hulk
D or O or Q = Unmasked Wolvie
E or MO = Unmasked Venom
F or T = Captain America
G = Red Daredevil
H or L = Yellow Daredevil
I or W = Human Torch (Reg)
K or S = Unmasked Human Torch
L = Storm (White)
M = Storm (Mohawk, black)
X = Storm (Mohawk/Silver with gold trim costume)
N = Storm (Black, full hair)
U = Variant Costumed Red Daredevil (a different red costume than the one from the first release!)

*NOTE: There are no new letter designations for the series 1 first release Daredevil or the 2 Storms (white full hair & black mohawk) because they were replaced by their new variants "X", "N", & "U" (^see above^).

Series 2 Mystery (11 figures + 1 variant Dr. Doom):
1. No hood metal head OR hooded unmasked human head Dr. Doom
2. G. Rider human
3. Mr. Fantastic normal hands
4. Spiderman half masked
5. Blade with disintigrating
6. Cyclops w/ hair
7. Nightcrawler w/ sword
8. Phoenix
9. Rhino
10. Thor
11. Namor

List of Mystery Codes for Series 2 (11 figures + 1 variant Dr. Doom): (Note: Letter Ingraved in back bottom left, Under Nightcrawler Pic.)
A. No hood metal head OR hooded unmasked human head Dr. Doom
B. Ghost Rider human (Johnny Blaze)
C. Mr. Fantastic normal hands
D. Spider-sense Spiderman (half masked)
E. Blade with disintegrating zombie
F. Cyclops w/ hair
G. Nightcrawler w/ sword
H. Phoenix
I. Rhino
J. Thor
K. Namor