I'm curious too. I've reviewed drawings and data reports for hundreds and hundreds of pieces of new and existing equipment and never once have I seen an ellipsoidal head that was anything other than 2:1 (when the corrosion allowance is excluded that ratio changes slightly but I've never seen any ellipsoidal head that wasn't nominally 2:1).
2:1 ellipse has a similar thickness to the cylinder. That is convenient.
A 2:1 ellipse is just a Tori head with 90% crown radius and 17% knuckle radius, compared to cylinder diameter.
You can order a Tori head with custom radii.
I guess you can ask anything that can be manufactured, and you need to pay the price. For uncommon dimensions you/manufacturer needs special equipment for the forming process. This will increase the cost a lot.
The common ellipsoidal head is 2:1, and almost all the elliptical head manufacturers have the forming equipment ready.
Your specification may be suitable for torispherical head with a proper crown and knuckle radius selected. However the thickness of if it may be higher than the 2:1 elliptical head.
I suggest talk to the specification provider for the possible typo.
Do you know the difference between sigma 1 and sigma 2 is in the photo?
Interesting that above 1.42 for a/b, the hoop stress becomes compressive near the knuckle and crown region (and above 2 the compressive is greater than the tensile stresses).