Cap ring references

Rims and Reasons
James H. Rose.
The Magazine Antiques, August, 1936, pp. 68-69.
Based upon his study of a great many pressed glass cup plates including the wonderful "foundering" USS Constitution, Rose argues that more complex mold designs had been used than glass scholars had previously imagined. Variations in the position and shape of the rims of otherwise identical cup plates could be explained by a separate cap ring.
Cup Plates of the Philadelphia Area
Helen McKearin and James H. Rose.
The Magazine Antiques, November, 1943, pp. 230-233.
An example of the use of cap rings to attribute a group of cup plates to the same, Philadelphia area factory. Four cap rings are shown in combination with six design-bearing plungers to create twenty four combinations of which the authors had located twenty.
Early Glass Pressing Technology at Sandwich
Kirk J. Nelson.
The Acorn, Journal of the Sandwich Glass Museum, Volume 1, 1990, pp. 38-50.
The author points out an important advantage of the cap ring - to simplify and reduce the cost of mold manufacture. Earlier he describes a previously missed technique for creating a variety of identically patterned forms by slumping or pressing previously patterned sheets of glass into unpatterned molds.