Thanks for looking. Having another think about the removable barrel idea. Twist lock/push button are used on others but they have a longer block and secure the barrel breech too.
Favour a discreet fix at the mo'. The land between the underlever fork will take 5 mm grub screws OK.
Any practical observations appreciated.
https://drive.google.com/file/d/1ZJr...ew?usp=sharing