Need some information about Mantinea 207 BCE

[D B Campbell]

Actually, I don't think Marsden was mistaken at all, since he directly quotes the greek, as I posted.

I know it's hard to believe, but even Homer nods. Marsden doesn't actually quote the Greek, Paul -- he just writes "stone-throwers (petrobolous mechanas)" in his text. (I've quoted the Greek below, as an attachment.)

I suspect your translation derives loosely from one of the french translations, who for "stones and stone throwing machines" have "pierres et tirreurs" ( lit: stones and shooters/launchers) - Shepherds english translation 1793 is rather difficult to get hold of !

Nope -- it's my own translation of the Teubner Greek text. I haven't seen any French translations of Polyaenus, although from your citation they seem accurate. Even Shepherd's version, though somewhat idiosyncratic, accurately conveys the meaning: "men expert in throwing stones, furnished with huge stones and pieces of ragged rocks for the purpose".
If you look at line 3 of the Greek text (attached below), you'll see that Polyaenus wrote "petrous kai petrobolous" -- no mention of machines!

The context also clearly implies machines ... Hand thrown stones are not going to travel from ridges to plain!...and hand thrown stones are hardly likely to outrange javelins either

If you try to envisage the scene, the men are dropping rocks on the Macedonians from above. There is no need to outrange javelins.

If you want to interpret Onomarchus' petroboloi as machines -- besides the difficulty posed by the fact that we don't hear reports of stone-projectors for another 20 years -- you must somehow get around the design flaw (inherent particularly in stone-projectors) that, shooting downhill, the missile doesn't wait for the bowstring! It starts to slip away itself.

You can see that arguing for stone-projectors here starts to become very complicated. If Marsden hadn't made the initial mistake, I'm sure nobody would ever have imagined that Onomarchus' stone-throwers were anything other than men dropping rocks from their vantage point above the Macedonians.

Charon and Isidorus both describe non-torsion stone-throwers throwing balls up to 40lbs. We don't know for certain that Philip didn't have stone-throwers, merely that Alexander introduced Torsion stone-throwers at Halicarnassus, and as I said there is no reason the Phocians couldn't have them.

A pretty good reason would be that they didn't exist at the time. Your evidence for non-torsion stone-throwing bow-machines comes from a writer who may have been writing at any point between 238 BC and 133 BC, so we don't really know when Charon or Isidorus built their machines. Also, we don't know what size of missiles they used, because Biton doesn't tell us.
Anyone who wants to see stone-projecting catapults at this early date really has to come up with some evidence. Because, so far, there isn't any.