Ernie1 also, I don’t know which 58.5mm tamper you have, but if I remember correctly, the one I had at the time had a slightly angled base and it was a tight fit. I didn’t have vernier callipers at the time so it might be even it was a 58.5 something rather than a perfect 58.5mm.
I currently use a VST 18g, I dose 18.5g in it and I have a 58.5mm (measured with vernier callipers) tamper and if it’s perfectly.
Edit: you posted above as I was typing this. 👍