Italian indicator System, an example of using:
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Suppose that on 23 November 1942, a Navy cipher clerk from Super Marina wants
to send a message. First, he chooses a External key not crossed in a Page 
dedicted to Super Marina of the current "Table of External Key for cipher 
Machine C38m" (Tabelle delle chiavi esterne, Ottobre 1942). In November, the 
Distribution list attributes eleven pages from this table to Super Marina 
(pages 3rd to 13th).

For example, he chooses the key "LCAECI" in page 09, column 2, row  14 (09214).
Then it crosses out this key to not reuse it.

On the 23th November, the masking code is 49025, then, the numeric indicator 
is 09214 + 49025 = 48239 (without carry).

If he wants a letter indicator , he translates each digit in  letter with 
this  simple code:

            A B C D E F G H I J
            0 1 2 3 4 5 6 7 8 9

Then, the 48239 indicator becomes "EICDJ".

He puts this indicator at the beginning and at the end of the message.

A simple challenge (with the same masking code 49025)
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      GCHFC BIAQP KUMAQ LGLYR BEISC KNDEC NIHCH GBSKT OYLAW ZWAYH
      ZOGFF ZISZM EAIUT WXJPU SXPEM FQWIM QWMXF JDIEF MCOIA GTGDC
      FUAHC GCHFC

(We have used the Lug and Pin Setting of the US Manual TM 11-380 1944)