I found a listing for grade 1.7784 in the reference book Stahlschlüssel. It appears to be an aerospace grade alloy steel, but no standard or specification was provided. The composition is as follows:
C = 0.38-0.43
Si = 0.80-1.00
Mn = 0.20-0.40
P = 0.015 max
S = 0.010 max
Cr = 4.75-5.25
Mo = 1.20-1.40
V = 0.40-0.60