Can anyone explain Arrow T-series staples and guns?

A local hardware store was selling off a handful of odds and ends, and I noticed a couple of Arrow staplers for $5. This is the all chrome, heavy stapler. It looked like one I already had, but mine was rusty and I'm always looking for it, so I figured this was too cheap to pass up.
So anyways the model number engraved in the stapler is T-25M, but the single-sheet exploded parts view inside the packaging says it's a T25P Gun Tacker.
The printing on the blister-pack packaging says I'm supposed to use T-25 staples.
It lists a bunch of other sizes that (I guess) are possible if you have the right stapler:
T18 (3/16" - 5 mm) T25 (1/4" - 6 mm) T37 (5/16" - 8 mm) T75 (1/2" - 12 mm)
I had been to another hardware store a week ago and picked up some T20 staples (for my other gun, which is the same model as this gun) and they work fine in this T25 gun (and these T20 staples are made by Arrow), but it's got me scratching my head wondering why the T20 size doesn't show up in the above list.
The Txx naming system seems to be trade-marked by Arrow, but I'm wondering what the hell it's supposed to mean, and what exactly is the difference between T20 and T25.
It always pisses me off that they don't indicate the actual width (in inches, mm, etc) with these staples. It's always according to this strange Txx code.
And why is it that only Arrow seems to make staples for these guns?
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*Those differences are the width of the staple for different size wires and the appropriate model stapler. There are different lengths in each size.
http://www.arrowfastener.com/catalog/supplies/staples
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John Grabowski full-quoted:

So why can T20 staples be loaded and used in a T25 gun?
Why does the package printing on a T25 gun not make any mention of the existance of T20 staples?
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">> *Those differences are the width of the staple for different size

*I don't know, but I bet tech support at Arrow knows. I'm guessing here, but maybe it is because the T25 is a cable stapler and the T20 staple is not made for cables. If you look closely at the packaging you will notice that the T25 staples are round headed while the T20's are flat headed.
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You didn't buy an ordinary stapler. You bought one dedicated to tacking low voltage wiring with round top staples.
--
DT



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DT wrote:

Well, this is my second T25 stapler, and all I ever use in them are the flat-top T20 staples.
I figure that if the staples fit properly in the feed guide, that's all you need to worry about.
I loaded a rail of flat-topped T20 staples into my new T25 gun and drove a few staples into some scrap hardwood. Worked fine - as I expected.
It's strange. Arrow seems to make guns with model numbers that match the staples.
So they have T25 round-topped staples for T25 guns.
I don't see a T20 gun to match the T20 flat-topped staples.
They do sell a T2025 gun, which claims to use both T20 and T25 staples.
But like I said, the T25 gun seems to shoot T20 staples just fine.
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