The 819M-BASE Base Plate is used to mount 819C or 819D Series integrating spheres. Its dimensions are 6 x 6 x 0.40 inch. It features 4 holes for ¼-20 screws with 5 inches apart for the Imperial breadboard pattern, 4 holes for M6 screws with 100 mm apart for the metric breadboard pattern, and a 2 in. long slot for an increased flexibility for the post/post holders of your choice.  Posts and post holders are sold separately.  Note that 819M-B4, 819M-B6, and 819M-B8 are being phased out.