Following the introduction of the Mark XX , a nomenclature policy was adopted under which new mark numbers were to be issued only to reflect substantial increases in psychotronic capability. (The new practice was temporarily abandoned with the Mark XXVII -Mark XXIX series, but resumed with the Mark XXX .) The Mark XXI actually proliferated into several sub-variants which would have received their own mark numbers under the old system, including yet another attempt at a 'heavy' Bolo (which tipped the scales at over 20,000 tons and was never adopted for field use), and the 'stealth' forward reconnaissance XXI/I. Mainstream Bolo development, however, followed the 'standard' format, and the Mark XXI/B reverted to the single 60cm Hellbore main armament of the Mark XIX. The major advance lay in