Just to keep things straight - it is not a 15-91, it's a 15-30. It's the "one version" earlier than the 15-88 to 15-91 series.
it has the old style bobbin wider, it does not have reverse
This is one of the early 15-91's
This is also 15-91 - the 15-90 is the same except for the potted motor. (88 and 89 are handcrank and treadle versions)
This is a 15-30
This is an earlier 15-30
and one more 15-30
BTW - what you have is way more collectable than the average 15-91