Adaptor backing flanges are specialized cast iron flanges designed to interface a glass end with a flange having a different bolt configuration. Essentially, they act as transition pieces when bolt patterns or flange standards differ. These adaptors are especially useful in connecting glass or PTFE bellows assemblies to non-glass equipment such as glass-lined reactors or metal process hardware. They maintain the mechanical support function of backing flanges while accommodating mismatch in flange geometry or configuration.