What is common to the Hebrew letters Kaf, Mem, Nun, Pe and Tsadi?

There are the Hebrew letters for decimal character codes.
Kaf = 20, Mem = 40, Nun = 50, Pe = 80 and Tsadi = 90

www.fourmilab.ch/etexts/www/hebrew/Bible/help.html

Hebrew is written from right to left in horizontal lines.

Some letters (kaf, mem, nun, fe and tzadi) have a final form (sofit), which is used when they appear at the end of a word, these are shown in the table below the normal form.

There are no separate numerals in Hebrew, instead standard western numerals (1, 2, 3, etc) are used. Some letters have numerical values

http://www.lingvozone.com/languages/Language%20Information22.htm

These letters are written in a different way at the end of a word.

"Both the old Hebrew script and the modern Hebrew script have only one case, but some letters have special final forms, called sofit (Heb. סופית, meaning in this case "final" or "ending") form, used only at the end of a word, somewhat as in the Arabic and Mandaic alphabets.[4] As can be seen in the tables given here, only five letters have a sofit form: ך → כ (kaph and khaph), ם → מ (mem), ן → נ (nun), ף → פ (pe and phe), ץ → צ (tsadi or tsade)."

https://en.wikipedia.org/wiki/Hebrew_alphabet#Letter_table

The "sofiyot" as they're called have also their own mnemonic values. If Aleph=1, Yod=10 and so forth, than Kaf Sofit=500, Mem Sofit = 600, Nun Sofit = 700, Pe Sofit = 800, and Tzadi Sofit = 900. Thus the 27 different characters of the Hebrew alphabet make for a perfect numeral system, with three sets of nine digits each (zero not being marked).