A large single sheet of graph paper (approx. 27" x 57", found folded to A4 size) showing "FC" numbers.
In 1961, Crick, Sydney Brenner, Leslie Barnett and R J Watts-Tobin began a series of genetic experiments designed to investigate the general nature of the genetic code by inducing mutations in the DNA of bacteriophage T4, a virus that infects bacteria. Following an initial series of experiments (published as Crick et al, 1961, "General nature of the genetic code for proteins"), sustained detailed work continued to provide a more comprehensive genetic map, eventually published in Crick et al, 1967, "Phase-shift and other mutants in the first part of the rIIB cistron of bacteriophage T4," Phil Trans Roy Soc, 252 (780), 487-560.
The large single sheet of graph paper that comprises this file is a working draft document from this more comprehensive mapping exercise.
For background, see also Crick, What Mad Pursuit (1988), pp. 127-36, and Judson, The Eighth Day of Creation (1996), pp. 442ff.