Some guns have on the loop a round indentation where the "J" spring fits into. Others the "J" spring is flat on the end and just catches on a lip of the loop. The one with the indentation are great because the "J" spring is now centered and makes the for-end go on easier.
Some "J" springs are straight, others bent and if the gun was not cocked before taking the for-end off, the "J" spring can be over-bent and will not hold the for-end on. The Curtis for-end was a savior to many for-ends not falling off and by 1919 was discontinued because of being an expensive part to make and hand fit. After 1919 all Field Grade guns without ejectors just had the "J" spring, all other grades without ejectors had the banjo roller release along with the "J" spring.