I see that the instrument response in RESP and SAC file formats is slightly different in the Poles and Zeroes section. In the SAC file, there is an additional "one zero." Is the SAC response file automatically converted by rdseed to displacement? Most instrument responses are reported in velocity, do all SAC responses have one additional zero?
SAC by default assumes that ground motion is in terms of displacement. Most of the IRIS-era data responses are given in velocity units, so, yes, rdseed adjusts the poles and zero file output for SAC so that the units are displacement. It also makes the required (2x pi) adjustments when poles/zeroes are given in hertz rather than in radians per second.