To Woody-iow , the fact thast you have acquired photos/ postcards with no copyright info on them does not exonerate you or the "supplier" from breaking the law or give you licence to print them. They are still (c) of the originator (unless he /she has given permission). You cannot assume permissioon is granted. There is no need for the copyright holder to add any marking / watermark to their creation. It is assumed to be their property at the instant of the camera button click.
See: the "Berne Convention 1886" and the "Copyright, Designs and Patents Act 1988" .. copyright arises automatically, no formalities such as registration or copyright marks are required.
There is no formal registration procedure for copyright; as long as your work qualifies for copyright by being original, it obtains protection as soon as it is in a permanent or fixed form. In UK law, originality is defined as using your own skill, labour, judgement and effort. In other words, what this means is that the work must not be copied from another work; it should originate from the author.
Then there arises (c) protection in other countries. However, signatories to the Berne Convention treat the (c) and recognise the copyright of works of authors as if from other signatory countries in the same way as it would recognise the copyright of its own nationals. This means that UK copyright law will apply to a work published or copied in the UK, although it may have originated in France.
If you want to use a piece of work that is still in copyright, you will need to seek permission from the copyright holder; acknowledging the author is also not enough. It is assumed a work is (c) unless there is written evidence to prove otherwise or time has elapsed (ie 70 years after death of originator)
I have photos purchased in the past from Skyphotos and kindly donated from companies such as Esso, BP, Shell over 50 years ago. I would be in breach of (c) if I loaded them on to Shipspotting as would Shipspotting if they were displayed; no money would have changed hands. The fact that I bought some does not confer on me any copyright.