The trouble is the old testament was written in Hebrew and the new in Greek. It seems unlikely they would use the same system. (It also sounds suspiciously like you're alluding to Roman numerals where L is 50 not 5000, and I can't see an "L" equivalent in Greek or Hebrew that has the value 5000.) It's possible this mistake happened when it was translated into Latin, but its more likely that what happened was distorted before it was recorded - much as rumours mutate today. The bible is "entertaining" gossip not factual accounts.
You are correct in pulling me up about the abbreviation "L". I apologise for that; it was my memory. The abbreviation was "lp" and thanks for pulling me up on that.
I found the gentleman's website and it is here where he wrote on this and it can be read here:
The part of the page in question is this part (copy and paste coming up: I apologise for this but I think that it makes for interesting reading).
Some commentators have persisted in the ridiculous notion that there were 30,000 Israelites in ambush behind the hill west of Ai where the combat engineers hid, for this is what the Bible account says. However, we need to understand that the King James version of the Bible is the result of translation from Ancient Hebrew into Aramaic into Greek into Latin into English! While the main thrust of the Bible's words are not in question, some of the detail has been muddled. For instance, we know that many of the ancient Egyptian scribes, when copying numbers, for some unknown reason seemed to add an extra zero to the figures being copied.
Worse, the copying in much of the Old Testament was done anciently in a form of shorthand, in which it was the practice to simply drop the vowels from each word. And herein lies a problem: the word for "thousand" was elleph and the word for "warrior" or, in the Bible, a "mighty man" - meaning a regular soldier as opposed to a conscripted levy or a militia man - was alluph, and if the shorthand was employed they both became lp! Which is correct in this case? 30,000 or 30? Let's think about it for a moment - would it be possible to hide thirty thousand men less than five hundred yards (485m) from their target town with wide-awake watchers? No: the noise of their collective wriggling and belching would be heard a mile away! It has to be just thirty. Anyway, how many men do you need to set fire to a small and empty town? Again, thirty seems about right.
On the other hand, when the record says that Joshua sent 5000 to loop around to the right and hide behind his own command position, it only makes sense if, indeed, there were 5,000; what effect would five men have had, charging into the rear of the Ai-ites?
What we are saying is that to understand the records - of any war - it helps to first understand the way the records were made up, the mechanics of the recording. (For the record - pardon the pun! - it is interesting to see that the same numbers problem affects the story of the Israelites under Moses trekking across Sinai after leaving Egypt. In the Biblical book of Numbers the total of the people is given as 603,550 - and that's just the men over twenty years of age, quite apart from their families. At Elim were twelve wells; that's an average of over 50,000 men assigned water from each well. They'd die of thirst just waiting their turn to draw the water! If, however, we apply the elleph/alluph rule the figures change dramatically. This now translates that were 598 regular soldiers and 5550 other men available as instant soldiers when the need arose. Now that number, with their families, would have been able to survive the water queues at Elim. As we said, you need to understand the way the recorders worked. It also helps if you know the type of person who wrote the record; if a copy clerk in a monastery is writing about the technicalities of battle he's likely to get it wrong here and there; we need to read between the lines he ignorantly wrote.)