I haven't used tinkercad but it would appear those two planes didn't actually join. I'm not sure grouping has that effect. Usually if I'm doing that kind of thing I use an extrude to join function.
This morning I needed a dummy / fired 38 spl case. Nothing on hand, so it was satisfying to make one rather than buy / wait. Anyone else enjoying making their own tools, speciality bits and pieces, that are right for the job ? rather than buying things ?
Generally pennies of those era had Britannia on one side and the reigning monarch on the other, so in 1873 it would be Victoria. Most english coinage has the reigning monarch on one side.
You may find printing effective forms a little tricky just because they're so small a radius. You may find more meats required for them to be strong enough.
You might well be able to print plastic ones that work though.