I have found out that there seem to be two different serial sequenses for Proto: s. Serials that start with a "F" and then numbers and serials that start with a "BCOC" and then numbers.
I have too few recorded numbers to make any assumptions in differences in build. Hopefully others here can provide theirs and we will reach some concensus in the future.
As of yet it seems like the early ones - having conventional cable routing thru rings on the TT are the earliest ones and those have "low" F: s (030 and 058).
To me known serials:
F030
F058
F136 (added 7/7 2016 found at "vintage art work" on flickr)
F.243 (added 5/5 2016 - and also have a dot as 254)
F.254 (has a dot between F and number)
F287 (added 16/8 2016 -found in old ebay auction)
F370
F382 (mine)
F403 the one in the PDF-file at
http://www.ciclimasini.com/images/90...i%20Caurus.pdf
BCOC157
BCOC360
BCOC515 (the O probably stamped at an angle - making it look like BCCC)
Example:
The F403 in the PDF is interesting as it has the brake cable routing both entering and exiting the TT on top, while mine with a lower serial has below/top (and also F370). This being maybe contradicting in a time line - if making the assumption that top/top would be an older standard more in line with pre "mandatory" under the handlebar tape routing.