Given "Black Onyx" (I think other name was 626--blackened action and choke tubes) had no x-bolt and some were made for 89mm magnum cartridges indicates that only Purdey double under-bolts are needed for sufficient strength. They could have made cross-bolt and no cross-bolt "GR" variants at same time for different markets. While in Anglophile countries x-bolts are not admired they are loved in places like Germany and Austria. "Fritzies" certainly love extra bolting.