“Main mass” is a scientific term used to describe the largest single piece of a specific meteorite. For example, the main mass of Clarendon (c), found in Texas in 2015, is a whopping 345 kilograms, but smaller pieces were also recovered and all of those are believed to have weathered off from it. We often think of a main mass as being very large, but they can be of almost any size, depending on how much of that particular meteorite was recovered in the field and how much remains intact today.
When a new meteorite is found and submitted for classification, a sample (typically about 20 grams) is removed and sent to the institution carrying out the analysis. In rare instances, the remaining piece may be kept whole and made available for sale to collectors as the official main mass. Such is the case with this specimen. It was recognized as a stone meteorite by expert Allan Lang and classified. It is accompanied by his original specimen identification card.