Here is a Sigma EM18. It's just like the Martin EM18's and made during the same time period. This one has the same Dimarzio pickups and Martin tuners. The weird thing about this guitar is it has no strap buttons and no screws to hold the back plate on, it is glued on.

Here is where the strap buttons should be. But you'll notice there are no holes or anything for any strap buttons...weird huh?