I would hesitate to do so.
All my Lindner and Schuler SBTs are about the same age as your Flues, but I shoot RSTs in them.
The Flues is not as strong as the Sextuple lock Lindners and that is one reason not to do it.
Second is that care must be given to the wood.
The low pressure loads are kinder to them and therefore sensible to shoot.
If the gun were an NID, I would shoot SAAMI loads all day.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.