Will the spool piece be open or closed? What is the construction material?

CP: the standard spool piece will be an open spool in stainless steel/painted carbon steel. Closed spool comes as an optional cost.
GP, GH: standard spool piece will be a closed tube with tail and top flange matching with valve and actuator respectively. The assembly will be in painted carbon steel. Upon request (optional cost), a relief valve can be installed on the spool itself.
GT, RC: thanks to the ISO connections, direct coupling with the valve is the most popular solution. Open spool piece in stainless steel can be added in case valve and actuator interfaces do not match.
KT: direct coupling with the valve, when matching with the actuator coupling details available in pub097-001. Otherwise an ISO adaption/spool piece are available.