Several aspects of this have already been discussed but not all of them. As written in many Bibles, χ £ ϝ or χ ξ ς, stands for 666. (616 is a variant.) Take those three current Greek numerals and turn the 60 (£ or ξ) down, put the 600 (χ) at the top left corner then place another 60 (£ or ξ) in the top center, followed by inserting 6 (ϝ or ς) to the left of the right bottom corner. Ironically, the result strongly resembles Allah’s symbol, which is shown below.
Strong’s definition of chi-xi-stigma (Six hundred threescore and six):
“χξϛ chx stigma, khee xee stig’-ma; the 22nd, 14th and an obsolete letter (G4742 as a cross) of the Greek alphabet (intermediate between the 5th and 6th), used as numbers; denoting respectively 600, 60 and 6; 666 as a numeral:—six hundred threescore and six.”
Chi (uppercase Χ, lowercase χ; Greek: χῖ) is the 22nd letter of the Greek alphabet. In the system of Greek numerals, it has a value of 600. Chi has no Phoenician equivalent. In ancient times, some local forms of the Greek alphabet used the chi instead of xi to represent the /ks/ sound. This was borrowed into the early Latin language, which led to the use of the letter X for the same sound in Latin, and many modern languages that use the Latin alphabet.
Xi (uppercase Ξ, lowercase ξ; Greek: ξι) is the 14th letter of the Greek alphabet. It is pronounced [ksi] in Modern Greek, and generally /ˈzaɪ/ or /ˈsaɪ/ in English. In the system of Greek numerals, it has a value of 60.
The Greek word for “mark” used is “charagma” which is translated as follows: “a scratch or etching, a stamp or badge of servitude” (Strong’s G5480). The Greek word for “right” is “dexios” which means one’s right hand or SIDE [which includes the arm] (Strong’s G1188).