Sorry was in a meeting when I replied earlier...
All you need is longer screws with the same thread pitch as the factory (non-spaced) butt stock plastic/pad, and then a SLIGHT modification.
On standard Rokstoks with no spacers they use the normal Stockys Phillips size 10 screws with coarse threads, these self tap into the foam/carbon which is why I mentioned before DO NOT overtighten, they strip easily.
On spacers added Rokstoks, they use a finer thread M4 Allen head bolt, with different lengths depending on how many spacers are installed.
So if you want to add spacers onto a "non-spacer" version of the Rokstok, you cannot use the M4 Allen bolts from Stockys and UM as they won't thread into the butt stock.
Instead of sending it back, just pull out one of your phillips screws and take it to the hardware store, and buy two that are long enough to get through your new spacers. There is no need to worry about diameter change as the Phillips Size 10 fits fine through the spacers access notches.
The slight modification...
The only "issue" is that on the "spacer installed" Rokstoks that first 1/4" spacer acts a "line-up tool" as there are small "nipples" on the spacers that you will either need to file down, or notch into either the factory butt pad or the stock end. Each spacer has these nipples to help keep the stack aligned and held together while screwing in. They are TINY, and I filed mine off, set the first one flat/lined it up on the butt stock and screwed the thing all together. No issues.
Difference in screw types.
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Phillips screws work in the spacers, just need to be longer.
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Standard Rokstok base with no spacers.
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Factory Spacers installed version, also showing line-up "nipples" for the spacer stack.
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1/2" worth of spacers installed onto a regular "non-spacer" rokstok as per this method. This photo was just mocked up, when I installed it I dabbed a bit of epoxy on the first spacer to hold in place for lining up properly, and to help hold it together should a screw strip out.
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1/2" worth of spacers installed from UM/Stockys as you'll receive if ordered.
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