Both my Smiths are 18mm spacing. Sometimes I play my other basses which are 19mm spacing on a 35inch scale. the 18mm actually feels more comfortable for me.
I have owned basses with 17mm or 16.5 mm - and those feel too narrow for me. I sold all those basses. I cannot go less than 18mm.
I perform with different brand basses (for years) - and now I find I can adjust to most basses within a minute or so of playing them; of course some basses (Smiths) are easier to adjust to
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