I know the 19E and 21A were extra strong models, and many of the 1950s models were made to take thicker thread than later on. They were made to handle the same as the cast iron machines, before polyester thread becamse the standard. Thicker thread and larger needles were expected. As you well know, cotton and linen thread are much thicker for extra strength, and slik thread is not practical for regular cleaning methods. That is why I dared comment on these models. My freearm machine now is a Bernina, so I couldn't check.