IMHO the use of patinated old coins, tested from different sides (or of different ages; whatever explains that the design is not the same) invalidates the test anyway, since we have no way of knowing whether it’s struggling to flow due to tip temperature regulation, thermal contact area, or surface oxidation. (And if they’re not the same age, they could even be made of different metals inside; low denomination coins stopped being solid copper long ago.)
If the test were with brand new coins from the same roll, it’d be representative-ish. Flat copper discs, all freshly scrubbed or chemically cleaned, would be far better IMHO.