Frequency Table for the Message

ABCDEFGHIJKLMNOPQRSTUVWXYZ
15131581320161630211327141915261393330171219201911

This clearly eliminates Cases 4, 5 and 6.

Referring to the recurring words and groups above noted, we figure the number of letters between each.

AII...AII45=3×3×5
BK...BK345=23×3×5
CT...CT403No factors
CTW...CTW60=2×2×3×5
DL...DL75=3×5×5
ES...ES14=2×7
FJ...FJ187No factors
NP...NP14=2×7
OL...OL120=2×2×2×3×5
OS...OS220=11×2×2×5
OSB...OSB465=31×3×5
PO...PO105=7×3×5
SQ...SQ250=2×5×5×5
TLF...TLF80=2×2×2×2×5
TP...TP405=3×3×3×3×5
UV...UV115=23×5
XMKU...XMKU120=2×2×2×3×5
UV...UV73No factors
YJ...YJ85=17×5

The dominant factor is clearly 5, so we may consider that five alphabets were used, indicating a keyword of five letters. Writing the message in lines of five letters each and making a frequency table for each of the five columns so formed, we find the following: