In the mid-fifties Thomas Barthel elaborated an alphanumeric transliteration system for representing the rongorongo signs, which is explained pp.38-42 of his Grundlagen zur Entzifferung der Osterinselschrift published in 1958 in Hamburg by Cram, de Gruyter.

This system is mostly numerical. A numerical code references each sign or variant thereof, while single-letter affixes express modifications of the basic sign (e.g. y when mirror-image, x when upside-down, h when superscript, t when subscript, etc.), and punctuation marks express how signs appear concatenated or combined, i.e.  a hyphen when juxtaposed (e.g. 4-4-4-7 ), a dot when linked to each other (e.g. 200.200.11 ), an apostrophe when fused together (e.g. 551'71 ), a colon when stacked above one another (e.g. 43:50 ), Barthel classified the signs into six main categories:

Signs 1-199	Figures seemingly representing objects, plants, or natural phenomena, some abstract.    The more common signs are assigned lower code numbers.
Signs 200-399	Anthropomorphic figures. Each digit expresses head, body, and hand shape.
First digit:	head shape.
2		Lozenge-shaped head, with a bulbous protrusion on each side, perhaps ears, or eyes, or coils of hair.
3		Round head in profile with gaping mouth, facing to the right of the reader.
Second digit:	position of the arms and legs.
0		Standing, both arms lowered or one arm raised.
1		Standing, both arms raised.
2		"Running" (laufend), any arm position.
3		"Kneeling" (kniend), any arm position.
4		Sitting, legs spread left and right, arms lowered or one arm raised.
5		Sitting, legs spread left and right, both arms raised.
6		"Squatting" (Schneidersitz), any arm position.
Third digit:	position and aspect of the hands.
1		Fingers inwards, similar to sign 61.
2		Fist clenched, similar to sign 62.
3		Hand pointing down, similar to sign 63.
4		Open hand, lifting, similar to sign 64.
5		Pointing up, similar to sign 10.
6		Fingers and thumb, similar to sign 6.
Signs 400-499	Miscellaneous figures with heads similar to the 300-399 series.
Signs 500-599	According to Barthel, reserved to anthropomorphic figures with miscellaneous head shapes, e.g. signs 510 and 524. Many, however, are not anthropomorphic at all, e.g. signs 518 and 543.
Signs 600-699	Figures with a bird's head, e.g. signs 610 and 619, but some are barely recognizable as such, e.g. sign 658.
Signs 700-799	Other zoomorphic figures.