David,
generally the most free hand Chribská pieces have flat bottom, same for Mstišov, ŽBS and Škrdlovice. Some pieces, however have pontil mark treeated as lense.
I have informationonly about practice in Škrdlovice, the pieces that had glass depth at bottom to small to make flat bottom was treated as waste and stored near glasswork chimney where later had been smashed personaly by Beranek. From this "chimney stock" glassmakers sometimes stole pieces that could be repaired by cutting, fixed it and use like "gifts" for doctors, magistrates, friends, etc..
I suppose that with Hartill (I have seen it written in CGR also as Hartyl) it is a bit different, fabricated piece had flat bottom when the thickness of bottom was OK, when not, they intendely not likvidated it as waste but cut the pontil mark to lense, becouse piece was relatively expensive.
I have also both bottom treatment types in my Hartill collection.
Jindrich