Hi Jeff, thanks for the question.  

This does look like a Magnesium deficiency.  The lower leaves being located on the bottom of the plant and the distinct interveinal chlorosis makes a good case for this deficiency. Low soil magnesium may be the cause especially in light sandy soil. High potassium can inhibit magnesium uptake as can a pH outside of a tomatoes acceptable range.  High levels of calcium with low levels of magnesium can also cause an uptake issue. Imbalance of the ratio of nutrients will cause deficiencies.  A soil test (Home | MSU Soil Test) can help.  Magnesium can be added with Epsom salts (see link). Check out the following links:

Magnesium Deficiency in Tomato and the Relationship with other Important Nutrients — Plant & Pest Advisory (rutgers.edu)

Monitoring and Correcting Magnesium Deficiency in High Tunnels (sdstate.edu)

I hope this is helpful.