It simply indicates when she was produced in the line
That she was made after the patent had been approved and after Hasbro decided to finally get around to removing that stamp from the toys.
Basically when MLP were first released and first went to production, the patent was still pending approval at the US patent office. So in order to defend the product, they put a "patent pending" notice on each pony which acts as to say "we are waiting approval of our intellectual property and rights for this product so dont go copying it". It also meant that after the patent was approved everything produced within the patent pending period was granted equal rights to everything produced afterwards.
After the patent was approved, Hasbro were then able to tell their manufacturers to remove the "pat pend" stamp. Two things are notable here. 1) It is expensive to produce new moulds just for one just minor change. So instead manufacturers would have added a small amount of molten metal to the mould to cover over the "pat pend" statement rather than produce a whole new mould. Thats why often it looks crudely covered up. And 2) Hasbro and their manufacturers took their time to get to this stage. The patent was approved in 1983 but due to manufacture typically being significantly ahead of stocking, it was not really until Year 5 that we start to see the pat pend disappearing.
All of your ponies from the same time as Art Time will have the pat pend section of the original mould covered over.
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