The format of the second photograph you've posted looks very much like the mounts employed for photographs produced from the roll film used in early Brownie cameras. I'm specifically thinking of the No. 1 and No. 2 Brownies released in 1901, but I'm sure many different cameras of the period used these mounts too. You haven't shown the full extent of the mount, but does the rest of it show an embossed scalloped pattern?