The Astro-Physics precision-adjustment pier adapter mounts directly to an AP portable pier, and rotates on a bearing surface for ultra-smooth operation. This allows you to lock the 900 mount down with no need to loosen the base during azimuth adjustment. The top plate of the adapter rotates through the 900GTO mount's full 15 degrees of azimuth motion. (It does not rotate through a full 360 degrees!) This option is especially recommended for use in the field.
The AP Precision-Adjust Rotating Pier Adapter includes two machined plates, four machined aluminum lock knobs with 1/4 ID x 1"OD flat washers, 1.9" azimuth adjuster block, six 5/16-18 x 5/8" socket buttonhead screws and 5/16 x 9/16"OD x 0.060" flat washers. All machined parts are black anodized and the fasteners are stainless steel.
More information on the AP Rotating Pier Adapter.