I..C.I Kynoch originally made their solid bullets with cupronickel jackets, except for the .416 Rigby (which they made with steel jackets as per the specifications of John Rigby & Co.). In 1956, Kynoch began to offer all of their solid bullets with steel jackets until Kynoch completely ceased manufacture of centerfire ammunition in 1972.
So any vintage English double rifle made AFTER 1956 is guaranteed to be safe with steel jacketed solid bullets. Because they were originally regulated for and proofed with steel jacketed Kynoch bullets. The employees at James Purdey & Co. told me this when I visited their London shop in 1976.
After 1939, John Rigby & Co. didn’t manufacture anymore double rifles until the 1990s. So the original Rigby double rifles made before 1939 were obviously NOT regulated with steel jacketed solid bullets. While they MIGHT be safe with steel jacketed solids, there’s absolutely no guarantee unlike their post 1956 counterparts.
@Ray B ’s 100 year old Rigby double is likely to be safer with Woodleigh Hydros than steel jacketed solids.