serries 4 trigger held by bolt and holding plate,the adaptor is basically like a mk1 but 15mm longer as per new type mk1 adaptor. as to have a standoff at its rear for the valve stem return spring to sit on.same valve body but needs a small front port of 4.8 mm to keep the power curve decent. made a few regs for these which helps them regarding shot count and no curve.
kevG