The ruins of the Rievaulx abbey, that sit in the plains near the River Wye in the north of England were the site of some of the greatest amount and technologically advanced iron furnaces in the world. 1997 a team of archaeologists, led by Rob Vernon and Gerry McDonnell of the Uni of Bradford, were investigating 12^th century iron making techniques, and discovered that rather than the standard small foundries, found a large number of technologically advanced sites, and the furnaces "in use during the thirteenth century at one of Rievaulx Abbey's iron smelters were producing iron at a level of technical sophistication of that of eighteenth-century Britain."

A fun little addition to this is the claim that Henry VIII's decision to close the monasteries in 1536 "delayed the Industrial Revolution by two centuries."

Source | Rose, W. THe most powerful idea in the world: A story of steam, (London:2010) Page 138

Not metal working specifically but between 1341 and 1359 the English monarchy managed to manufacture around 1.25 million arrows for their army. From what I remember reading there was a mass programme of teaching peasants to be able to use bows, and they built up a massive store of arrows so should they have needed them they could have literally put a bow in every hand and sent them off to war with enough arrows to supply them.

source: Wadge, Richard (2007). Arrowstorm. Stroud: Spellmount. pp. 160–1. ISBN 978-1-86227-388-7

There were various industries that had long been reliant on powered equipment, in particular ones reliant on mechanically simple repetitive actions. Cam Hammers (aka trip hammers) and drop hammers were used in paper making, blacksmithing and for crushing ores, blast furnaces used water powered bellows, and sawmills were water powered as were flour mills (such as the industrial scale Barbegal mills in Roman-era Gaul). Windmills e.g. those pumping out the Dutch polders might also be of interest.

There are aspects of mass production you can find kind of early, perhaps the simplest being that of specialization. To take the example of a flintlock fowling piece; if you have one guy who only forge welds barrels, another who forges lock parts, another who files lock parts, another inlets the wood stock for the hardware, another who does the decorative carving, another the inlay work, another who does the engraving, another the brass casting, the shop produces many more pieces than if eight gunsmiths were each trying to make an entire gun, because they can concentrate on developing certain skills ( and the master of the shop can hire men who have certain skills) and do just one kind of task. But this would happen in places with a big population, where there were markets big enough to support large shops. So, London, England in 1760 could have large shops, but Emmitsburg, Maryland, no.

Might be hard to draw the line as to what constitutes mass-production but I always found the advent and following centuries of progress with movable type pretty fascinating. The growth of print has some very interesting and serious implications.

"Riggng blocks" in sailing ships were "mass produced" in that assembly lines existed where manual labour cut and carved the appropriate blocks. Isebard Kingdom Brunel father actually managed to build a machine that could do that work and suddenly put a lot of people out of work as the Admiralty liked the cheaper price and higher output of the machine.