I saw you mention sls and wondered what you understood of the term then.
The front end you are describing sounds like a 1966, 1967 or 1968.
The 1968 Thunderbolt had the older brake but the twin carb versions had the different fork sliders and tls brake.
You may need to either change the entire front end to go to the 1969/70 'Triumph' style forks and brake.
If you have a rod dampened set-up as I suspect then you could also have the option of going to the 1968 sliders with 68 or 69/70 tls brake.
The easer and more original option would be to find the later front end.
Your axles screws through the forks and hub with a left hand thread and pinch bolts.
If you place your finger directly below the axle, at the bottom of the slider... cam you feel a hole each side ?
About 6mm at a guess.
If you have the holes then the forks should be rod dampened. They are well sought after in their own right.