Confused on difference between SC 55A and 56A ringers...

[RotoTech99]

In some of my earlier posts regarding the SC 55A ringer and 56A ringer I noticed the 55A is described as a 1400 ohm straight line ringer, and the 56A as a biased straight line ringer, also at 1400 ohms.

What are the differences between them; I noticed it is listed as a ringer for the 1212 set for metallic or grounded ringing.

Are there any photos or drawings of the SC 55A that might help me understand; I am guessing one lacks the biasing spring, but I am not sure otherwise.[/size]