S&W 1940 ID Help

[Banny]

Hey boys,
I live in Hong Kong and last week I went and checked out an old Canadian defensive position. I found quite a few .303's and even a Grenade Launching Cartridge. Anyhow, I also found a small buckle looking thing. It was in pretty good condition so when I first saw it I thought a hiker had dropped it recently, but just before I launched it into the Stratosphere I noticed an engraving:

S&W 1940

So it is most likely off some sort of a Smith and Wesson holster or ammo pouch. But I have been searching everywhere and I can't find anything similar.
The jagged edge is quite distinctive, but nothing I've seen matches.
The bottom part looks as though it was attached to the holder, while to top half is where the strap went through.

it is about 2cm square.
I have found plenty of other, much more interesting, items but this is annoying as I haven't been able to ID.......yet.

Any help would be greatly appreciated.
Cheers.

 

[AMMOTECHXT]

To me, the thing it most resembles is the clip that attaches to the strap on a steel helmet, for adjusting the tightness of the strap. XT

 

[hangarman]

Its from a British or Canadian Respirator. The elasticated harness is connected to the rubberised facepiece through these buckles.
Hangarman

 

[Banny]

Thanks

Thanks for the lead.
I managed to google a photo with what appears to be exactly the buckle I have, where else? but on another posting from this website. Top Stuff.

Its been annoying me for a couple of weeks now. I'll have to look into what the "S&W" means.

Appreciate the help. Ta