One of the things about the DS-24 brake valve is that it could be configured in so many ways depending on what portions you bought for the stack. It's been so long, I had to look this up in the WABCo book. Facing the brake pipe cut-out cock's side: directly above it is the full release selector cock, which was used to chose whether the brake valve's Release position supplied full main res pressure to the brake pipe or just feed valve pressure; on the right hand side of the valve is the safety control cut-out cock (the overspeed); on the back of the valve is the first service position cock, which is used to cut-out first service and maintaining functions.

In most locomotives, the SP GPs and SD-7s and 9s at any rate, the when the engineer sat in the seat, he essentially faced the corner of the stack which is between the brake pipe cut-out cock and the overspeed cut-out cock.

If you had a brake valve with the in-house modification for pressure maintaining, which consisted of plugging a port in the rotary valve's seat (the brake valves rotary valve, not the rotair valve), the the first service position cock was removed. The SP wasn't the only RR to go this route (the MILW did too), and WABCo didn't like it because they couldn't sell more apparatus (and charge to repair it).