For an ultra-narrow band pass filter, it would be better to base it on the "Spectral Delete" effect. That has extremely steep, linear phase filters that will give a much sharper cut-off.
However, let's go with the older BPF for now.
"s-in" is not a "special" symbol. It is just a normal variable and an abbreviation of "signal input".
In "version 1" plug-ins
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In "version 4" plug-ins, "S" is no longer a special symbol. The special symbol that now holds the audio data is the symbol *track* (or *TRACK*).
It is highly recommended to use the latest syntax version, which is currently ";version 4".
So you should use the ";version 4" header, and change occurrences of "S" with *track*.