from a quick google search..not sure if it really explains it, and an excerpt from another search..
>I recently found an old print in a antique (junk) shop. It measures approx
>2.5 inches x 3.5 inches. It appears to be a ferrotype on paper. The reverse
>is a "post card" type back. In the upper right where the stamp would go, it
>says: "Mandel's Positive Process-No Negatives" there is also a line drawing
>of what I assume to be the process camera. It looks like an old lantern
>slide projector. At the bottom it says: "Copyright 1911 By The Chicago
>Ferrotype Company". All of this text appears to have been applied with a
>rubber stamp after )I assume) the print was made. The paper is
>approximately the weight of modern double weight silver paper. The image is
>that of an average family sitting on a porch (a snapshot) so it's not like
>it was a commercial postcard of the Grand Canyon or something.
>Two photo historians at the local university looked at it, one specializes
>in non-silver processes and the other in 19th century photography, neither
>of them had seen one before or had heard of the process. Both did agree
>that it appears to be a 'paper' tin-type.
Tell them to check Melanograph and Monobath processes in my Encyclopedia
for a clue. They are fairly common.
I don't have my databases at hand but I am about 99% sure that this was a
direct reversal print, popular early this century with itinerant
photographers. It may also refer to the type of camera specifically used
for it. I'll check tomorrow.
Fredericton, New Brunswick, Canada
Matt's Photo Site
"I invent nothing, I rediscover". Auguste Rodin