S2s use threaded pills, but the entire head isn’t threaded. You can only have the pill so far forward. For this reason, S2 triple builds usually use spacers to allow the triple optic to get to the front of the light. OP wanted to make a light that made more use of that space in the head, so they filed down the threads so the pill could go further forward, which eliminates the need for a spacer and allows them to use an 18650 cell with a body tube that’s usually only able to be used with an 18500 cell.