B3 for 19 mm MicroMounts (with the flat ledge),
B3A for 19 mm MicroMounts™ (with the conical ALS-style shape)
B3S for 17.6 mm MicroMounts (with the conical ALS-styleshape)
Base styles B3/B3A/B3S have a copper post extending up from the stainless steel base, with a taper to capture MicroRT capillaries. Compared with B1-style bases, the tubing need only be drawn a very short distance past the sample before being captured by the post, and so slipping MicroRT capillaries on and off is much easier. The copper post can also reduce ice build up in the cryostream.