The description of the item is, along with SIZE, is:
18th Century Brass Signal Mortar. 7 1/2" high, weight 25 lbs., bore approx. 1 1/8". Just minor and insignificant dings, VG overall condition. Superior quality and condition to the average one of these. I have seen this type also described as powder testers for artillery ammunition. A good, original example.
My comment: Thundermugs were never "powder testers for artillery ammunition."