This is the magnificent well known William Baker hybrid Dyckia hb Mercury

 William Baker or Bill for those who knew him did a master job for Dyckias to be kown all over the world. He dealt with ornamental plants in LA California and owned a garden shop. He was a pioneer in Dyckia keeping and he used to sow Dyckia seeds. Those seedlings trays were usually sold. He would pinch out some seedlings he considered promissor or nicely looking and some of the most beautiful Dyckia hybrids came out from this practice. Bill didn´t know accurately what plants where the  father parents and this didn´t bother him a bit.

The years 70s,80s and 90s were invaded by Bill Baker´s hybrid and most of te more nicely looking Dyckia hybrids have some Bill Baker´s  hybrid blood in them all.

The most famous Bill Baker Dyckia hubrid is Dyckia hb Brittle Star.

Bill´s Dyckia species may have been Dyckia marnier-lapostollei, Dyckia fosteriana, Dyckia brevifolia, Dyckia platyphylla as we can spot some of their looks and characteristics amid 
Bill´s hybrids. Bill Baker was a foundation stone on Dyckias!

This is Dyckia hab Mercury BB

We can clearly see fosteriana looks on it.

Dyckia hb Mercury blooms accordingly to Dyckia fosteriana rules in time and form and shape.

What a bout crossing this marvel with this?

We could get a smaller well straight super white leaves. But this is guessing.