I will have to get all long-winded on you...
Multiple numeral systems are found in Phoenician texts, and although numbers are always spelled out in our current ashurith ("Assyrian" script, or "squarehand") text, it is likely that numeral-symbols were used instead in the pre-Captivity "Paleo-Hebrew script" texts: Phoenician and Hebrew were not really separate languages but just dialects of "Canaanite"; the scripts used in surviving fragments from the kingdom of Judah (the "Gezer alphabet stone", the "Hezekiah tunnel" inscription, the Lachish battle-dispatches) are all quite similar to the Phoenician rather than the ashurith (such scripts were revived under the Hasmoneans, but failed to catch on long-term; only the Samaritans use a kind of Paleo-Hebrew script now). The "gematria" style of numerals (first nine letters for 1-9, next nine for 10-90, filling in the 100-900 set with specialized letters) did occur, but "Egyptian" style, with multiple repetitions of glyphs, each glyph for a different-sized unit, was more common. Unfortunately there was little uniformity in the glyphs used, or their assignments.
There are several cases in the Tanakh best explained on the assumption that the editors of our ashurith text were confronted with numeral symbols in the original which they had difficulty interpreting. There is a Kings/Chronicles parallel (sorry I don't have the cites handy) saying "600 chariots" in Kings and "6000 chariots" in Chronicles, evidently reflecting a numeral written as six glyphs, with the editors uncertain whether that glyph meant a hundred or a thousand and making two different decisions. There is the wonderful sentence in Samuel, "Saul was a year old when he began to reign, and reigned two years in Israel" (very precocious toddler! and very busy, too!). Easiest explanation is that the reign-length was something like "XVII" and the editor knew what the "II" was (repeated single-stroke marks are practically pan-human for the low numbers; China writes them horizontally instead of vertically but that's as much variation as you find) but wouldn't hazard a guess about "X" or "V" (or whatever the glyphs were); and that the age was in round numbers with no units-digit, so the editor just left it blank. (Like the garbled sentences in Samuel where it seems some words were illegible in the original text the editor worked from, this is a GOOD sign for the basic integrity of the text: the editor is giving what he can read, faithfully, and refusing to invent any gap-fillers.)
There are also cases which make better sense if it is assumed that numeral glyphs for the base-seven time-units (pervasively referred to throughout the Tanakh) have been read in base-ten. Moses died at "120" years old after "40" years of wandering in the desert? Perhaps instead he dead at one jubilee and two weeks (64 years old) after wandering for four weeks (28 years). Then he was in his mid-thirties, not pushing eighty, when he challenged Pharoah: a more typical age for a rabble-rouser! Abraham was "100" when Isaac was conceived: or one jubilee (50). Sarah's age was tish'iym "lacking (plural)", interpreted as "90" (lacking ten, from a hundred) but could be a subtractive numeral (like "IX" or "XC" in Roman numerals) for "lacking one week, from a jubilee", that is 43 (as opposed to six weeks = 42). And this is now a normal human story: of course a childless woman who stops menstruating at 43 assumes she has hit menopause, and laughs at the suggestion she is, instead, finally pregnant.
You will, of course, say that these interpretations are speculative. But there are some clearer cases. The interval from the sack of Jerusalem to the fall of Babylon is conventionally described as shiv'iym "70" years of Captivity, although the archaeologists date these events as 585 and 536 BCE (seven weeks apart). The interval from the Exodus to the foundation of Solomon's Temple is given as "480" years: it is odd that it would be expressed as four jubilees and eight weeks = 256, rather than five jubilees and six years, but this interval makes chronological sense. The city-sacks in Canaan were around 1200 BCE, not 1450; the Hebrews in Egypt were said to have built the city of Ramesses (founded by Ramses II); the Philistines' arrival (under Ramses III) is just after the Exodus (the city called 'Azzah in Exodus becomes Gaza early in Judges; the Philistine language had no glottal stop). The cross-correlation between the reigns of the kings of Judah and Israel is notoriously difficult; reading the numbers in base-seven clears up contradictions (though not totally; some problems remain).
