Does anyone know what this metal fastener in the corner of an old wood picture frame.  Also, any idea to what era it was possibly used?   See attached photo.
Thx everyone!

That serves to keep the frame square. I don't think they are used these days, but it wasn't so long ago that they were common. For extra strength metal wedges are pressed into the wood across the corner join.
Similar to these-

I was told by some one a long time ago that it's called a ripple nail.  May not be the right name but it always sounded right to me. Still available in at least one place where I live. If my old memory bank is correct there were two different sizes.

Yippee!  Thanks to both the responses, I was able to gather enough key words to lead me to this: 
It's a "corrugated fastener"   --- A small wavy (corrugated) rectangle of steel with one edge sharpened so that it can be driven into wood to hold a joint together.  Also called a "wiggle nail".
Thank you!   

I used to work at a cabinet plant and used these with an air gun building frames. That was 20 years ago so don't know if they're still being used.

do you need some?  I have a drawer with some in, haven't looked in it for years. could be a dozen or a 112?

Hello, i knew I had a box of those around here. These were sold by Stanley, while they were still in the U.S., the label reads Corrugated Nail. For general repair work, picture/window frames, or the old wood storm /screen door. I had a hand-held driver for these, makes installation easier, that's around here somewhere too, these can be finger busters!! Regards, Lou

I sold them forever in the hardware store. they were used to repair  frames, window sash, and anywhere 2 pieces of wood came together.

Also known by a less politically correct name after a religious group.

A similar part known as a "Skotch Fastener" was popular too (note spelling)

If you ever have to remove one, don't; just scrap the thing it was in.  They're a bear to remove, and, when I've tried, I've always wound up destroying the wood they're in.

Like a lot of other "quick fastener" designs, they're not much good unless the joint is well-machined and tight.