This mantle helps prevent vapor condensation in the upper half of a flask during distillation of high-boiling materials.  The tops are designed for use with Series M, TM, or STM mantles.  They protect the flask, the solutions, and the laboratory in these ways:

1.  from drafts, which would disturb the reflux ration during fractionation;

2.  from flying glass of flask implosions in high-vacuum distillations;

3.  from falling objects

4.  from thermal strain between top and lower half of flask during high-temperature operations.  With a glass-fabric construction and laced-side opening, the elliptical hole accomodates 1, 2, and 3-neck flasks.