What's the difference between a mould number and a model number? 

Essentially, model numbers are exceptionally useful for identifying a Pyrex item, and mould numbers are completely useless.  Usually two pieces with the same shape & size will carry the same model number, but they are unlikely to have the same mould numbers.

These 1410 mugs are older than most.  Although they are made of Pyrex, they are branded with the company name, "Corning" instead.  This shape is normally numbered 1410, but these examples are not marked that way.  Instead, only their mould numbers are present, "17" on the left and "12" on the right.  These numbers have no significance.

A mould number is only useful during manufacturing.  If a mould develops a defect that is evident on the articles that are cast in it, having this number imprinted on the article makes it much easier to identify which mould needs repair or replacement.  For a product that is successfully formed without defects and shipped out of the plant, its mould number is irrelevant and holds no information for the purchaser.

Mould numbers can be numeric, alphabetic, or alpha-numeric, and can be one digit or more.  It might look like "5", "17", "0E", "C-1", "B09", "T", "Y-E", "AG", "D-FF", "AP-C", "J-38", "H X", "JJ5", or even another format.  Model numbers also can vary in length and contain letters and numbers too.  Experience is truly the best guide in this case, and there is no single rule for distinguishing the informative model number from the irrelevant mould number.

Oval shallow non-divided dishes were called 053 in catalogues, but this model number never appears on them.  A meaningless mould number does appear on them, "25" on the left and "29" on the right.  Divided dishes from the 1950s & 1960s often do not carry a model number either, but they are known as 063.

Very old clear Pyrex, as old as 1915, might not carry a mould number.  Alphabetic and alpha-numeric mould numbers are often found on reasonably old clear Pyrex, 1940s or older.  Clear Pyrex and opal Pyrex manufactured since the mid 1940s usually has a one or two digit mould number, often between 1 and 40.  There are numerous exceptions though, and this is not a rule for judging the age of a piece of Pyrex.  Instead, when the age of the item is already known by analyzing its shape or style, this guideline provides an expectation of what format the mould number might take.

Model numbers are usually three digit numbers, but there are some two-digit and four-digit ones too.  At one point, an 8 oz measuring cup was known only as "8".  Often a model number has an alphabetic suffix, necessary for items that are comprised of multiple parts, like a casserole or percolator.

On the right side, near the bottom, 54N is the model number for a 4 oz narrow neck baby bottle.  The "16" on the left side is just a mould number.

The most common suffixes for model numbers are "B" (bowl), and "C" (cover).  Others include: "H" (handle), "GC" (glass cover), "U" (upper), "L" (lower).  The latter two pertain to double-boiler pans, and "U" can also mean under-plate in the case of gravy sets.  Baby bottles can be suffixed with "N" (narrow neck) or "W" (wide neck).

Interpreting model numbers can yield a great deal of information about an item.  Often the last digit is related to the capacity of the piece, and groups of similar shapes frequently share the same first digit or digits, linking them together as a family.


Related Articles:

Pyrex Model Numbers 
Which model numbers are duplicates?
Clear Pyrex 1915 to 1950, handles & knobs
Dates for Pyrex patterns/pieces: 1940s to 1950s, 1960s to 1980s