Hello Andrew,
Just to clarify, the 2990 ref is actually an Omega Ranchero. Some were sold with seamaster dials, most did not sell well due to the name.
Many were sent to the Swedish distributor and became what is now known as "transformed Rancheros". The ranchero dials were replaced by seamaster dials with a very distinctive rounded logo.
2990 - small seconds
2996 - center seconds.
If you search for the following keywords you will find a lot has been written about them.
Omega Ranchero
Transformed Ranchero
Seachero
Belgian Ranchero - a model sold in Belgium in 1976