This picture above is one of my Japanese Black Trifele tomato seedlings, and you can see here quite clearly that it is not like other tomato plants. A little research on the internet shows that there are many hundreds of tomato varieties which have potato leaves. Big Boy and Brandywine are two with which I am familiar. There are lots more out there.
In comparison this is a regular leaf seedling. Some research has shown that the potato leaf (PL) varieties are slightly darker and thicker than the RL varieties and this is thought that it might make them more tolerant to disease. Interesting. I'll come back to this subject when more leaves have developed. Lovely bright, hot sunshine in London recently and these tomato seedlings have been spending their days outside, hardening off.