Keith, thank you very much for your reply.

There are at least two caseback variations on the Ref. 13-1.

First variation:
text in the middle of the case back, reference number written as „13 1“, same as listed at the OTD reference table.

See here:

http://www.onthedash.com/chronograph/reference-13-1-chronograph/

Second variation:
text on the outside of the caseback, reference number written as „13/1“

See here (also note the difference in the dial design at 3 o´clock):

http://chronocentric.com/forums/chronotrader/index.cgi?md=read;id=86794

This one also had the same caseback with the 13/1 (the seller provided me with a picture of the caseback):

http://chronocentric.com/forums/chronotrader/index.cgi?md=read;id=94691

You can find a lot more of the second variation on the net but I show only the ones I found at OTD.
I can provide the links to other examples with the second case back style, but I think the moderators should decide about this.

Anyway, all the visible information I found leads to the following conclusion for the 13-1:

- two caseback designs

- two dial designs

Greetings,

Hermann