1969 Swinger 340 distributor questions

redfastback said

for your #5796, i have that the distributor is for a 318. a 340-man is 2875783/ ibs4015b and ibs4015bs, and 340-auto is 2875780.

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Thanks Red. At least I have a dist number to hunt for.
What does ibs4015b and ibs4015bs mean? I don't recognize that at all. Sorry for the newb questions.

George