oops!
As I mentioned above I often put valve replacement additive in my tank (Every tank until the bottle is empty then a few without until I get remember to get another one). As far as I know I don’t have hardened valve seats. I have no odd noises (at least not from there) and haven’t seen any changes when I’ve looked at valve clearances. The engine pulls nicely but I never try to go above 60. So I’ve been quite happy thinking all must be well. Now having see your photos I’m not so sure. What a robust design indeed!
Chris
Chris