The Answer is rf
its a text sequence of numbers:-
one two three four five
till f of five
& divide it in pair of two it is the given sequence

Numbers after 19 were written vertically in powers of twenty. For example, thirty-three would be written as one dot above three dots, which are in turn atop two lines. The first dot represents "one twenty" or "1×20", which is added to three dots and two bars, or thirteen. Therefore, (1×20) + 13 = 33. Upon reaching 20^2 or 400, another row is started. The number 429 would be written as one dot above one dot above four dots and a bar, or (1×20^2) + (1×20^1) + 9 = 429. The powers of twenty are numerals, just as the Hindu-Arabic numeral system uses powers of tens.