WPI Stereotaxic Holders and ComponentsFeatures Standard Probe Holders include an 11mm square acrylic block connected to our standard 19. 5cm standard shaft (6mm diameter) with grooves spaced 1 mm apart. The grooves (on three sides) facilitate loading of probes. Options Order code Description 505249 Standard Probe Holder Corner Clamp 505250 Standard Probe Holder Side Clamp 505252 Electrode Holder, 'C' Clamp 505259 Replacement Corner Clamp 505260 Replacement Side Clamp 505261 Replacement