I don't have BS 592 1950, but the details are contained in BS 3100: 1967.
BS 592 Grade A was:
0.25% C max,
0.9 % Mn (max),
0.6% Si, (max)
0.06% S&P.
Ni 0.40*
Cr 0.25*
Mo 0.15*
Cu 0.30*
*residuals not to exceed 0.80%
UTS min 28 tsi
Yield min 15 tsi
El 22%
izod impact value 15 ftlb (either an impact or a bend to be carried out).
All castings to be supplied in heat treated condition. The heat treatment shall be carried out at a suitable temperature to give the mech properties required. (not very helpful).Perhaps this might help TVP.
Brtish Standards don't carry fracture toughness data in their documents. Unfortunately, we have no data for this material.
Alternatively, you might want to give a good casting producer a ring, or the Institute of Materials or a university engineering dept. Try UMIST.
Andy