Bspt X Npt Nipple

That's exactly the situation this fitting handles. One end is a 1-1/2" male BSPT threaded R 1-1/2 - 11, and the other is a 1-1/2" male NPT at 1-1/2 - 11.5 NPTF. Both sides are the same nominal size, so you're bridging two different thread standards without stepping up or down in diameter. Steel construction, straight profile, and a 51mm wrench flat give you enough to work with during installation.

The wrench flat measures 51mm, so you'll need a wrench that size to tighten it properly.

For that exact thread changeover, this nipple is the match. One end is 1-1/2 inch male BSPT (R 1-1/2 - 11) and the other end is 1-1/2 inch male NPTF (1-1/2 - 11.5).