I would say that it is mid 1850s to early 1860s. How lucky you re to have one in such good condition.
I think that there are a few threads on the care of them. I'll do some searching.
Re photocopying - how large is the plate. It might be better to make a really good scan and file it away. You could either have it printed professionally or print it out yourself.
I'm sure many of us would be keen to do a restore of it as well, if you'd like

Add -I'm glad I agree with Jim
