Even the Alpina factory records are (slightly) ambiguous but the accepted number built overall is 370 - all cars, LHD or RHD, were assigned sequential build numbers and your build number just means that your car was #185 out of the 370 built.
According to my records your car, #185, would have been the 86th RHD car assigned a build number. It was part of a batch of Z4s assembled at the BMW plant in Spartanburg, South Carolina in June 2004 and subsequently transported to the Alpina plant in Buchloe, Germany for dismantling/conversion to Alpina specification. This takes some time and, after conversion, it was then shipped to the UK and had it's first UK registration in April 2005.