Thse holes don’t have much
Thse holes don’t have much purpose for glass fusing. They rarely line up with what you want to see so they make lousy peep holes. They help with venting but only slightly. There was propbably once a conical ceramic plug that fit it – but you can jam some used fiber paper in or just leave it open. Heat loss by convection (air movement) at fusing temps is minimal since the air is so expanded and thin.
As for the stained glass, you can:
1) usually fuse a piece to itself
2) slump single pieces (lots of nice hankerchief vases
3) start testing pieces to see if you can discover which are compatible with each other
Keep in mind that a lot of stained glass doesn’t behave well when heated (ugly color shifts, lots of devitrification, even streaky glasses falling apart).
Many people find that using it ends up being more trouble than it is worth.
Helios Kiln Glass Studio