Thanks for putting that image up.
I have never seen that on a fuse before.
Your best bet is to go to the shop where you get the fuses from and ask for the fuses packaging.
Most of these little 5x20 fuses have only a T or no marking near the voltage rating which indicates normal blow.
As Nigel sais , just stick to the spring type fuse.
You can do an actual current measurement and see what the actual inrush start current is when your tube amp is cold.
You may need to add a low ohm series resistor in series with the caps to reduce the start up current which gets bridged out via a relay after 5 seconds or so.