Hi Common soldier,While there are far more detailed depictions of the aristocracy rather than the middle class in higher quality late medieval paintings and sculpture, there are a large number of survivals of belt 'findings' (buckles, strap ends, and decorative washers - even lengths of belt leathers with some metalwork attatched) in the archaeological record. From their quality and type, ranging from pewter or brass, up to gilt latten and silver, simple to elaborate in decoration, they seem to run the range of the middle class.
The Museum of London has a book on the subject, with a fairly large number of entries from the 12th century to the year 1450. That would be a good place to begin your search. from the number of survivals of decoration, decorated belts seem to have been commonplace, even amongst the middle classes.
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Bob R.