No fitment difference between left or right. BIG difference in quality. The "Anchor" or "Phoenix" ones are utter garbage.

For longevity I fill them with polyurethane.

If you do fill with polyurethane don't go higher than a Shore 40 hardness and a rating of 30 is preferred.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now