It wasn't a weapon per-se, but the Showa Mecha's forcefield repelled EVERYTHING that was thrown at it.  It could even No Sell Godzilla's Atomic Breath, which was a feat that Heisei and Kiryu failed to duplicate.  The closest a weapon on Showa came to Kiryu's Absolute Zero Cannon or Heisei's Plasma Grenade was its eye beams, which I think were called Wave Motion Guns.