Yours are typical holders for dry plates and (dry) sheet film. Vintage wet plate holders are very hard to find, since the cameras on which they were used are also very rare (before c 1860). The wet plates did rest on metal (mostly silver) threads in the corners of the plateholders, and did not rest or at a minimum on wood.
here's a picture of a vintage holder for wet plates - they have a little door at the back where you insert the plate:
almost all have stain on them from the liquid silver nitrate ....
Nowadays people make workarounds with newer / plastic holders: