Hmmm, the plot thickens, lol. This 2nd pic you've posted of the other slide shows a correct type for the '78-'79 carb set. It has sort of a golf tee shaped plastic bit held in by a snap ring to retain the needle .....
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So, it would seem you have mismatched slides, probably slide needles as well. The needle used in the '78-'79 slide, the 5O2, is longer than the earlier needles because it sits higher in the slide. It has to be longer in order to hang out the bottom roughly the same amount. Here's the BS38 needles compared. As you can see, the '77 and earlier ones are shorter than the '78'79 needle, tapers are different as well .....
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