I believe it is a 15-91. See how the the tension adjuster & thread uptake are on the side with the faceplate? It also has a vertical bobbin case. The 15-91 is neat because you can drop the feed dogs & do FMQ with it.
A 99 has the tension adjuster on the front (if you are facing the machine like you are sewing on it) & yes it is a 3/4 sized machine.
p.s. - I'm looking to get one & have compared soooo many pictures that I've got at least these two models distinguished from each other.