NSG, a long-standing partner of Pilkington, is understood to have wanted to consolidate its holding to demonstrate that it could supply glass around the world to Japanese carmakers.
There may also be closer co-operation on research and development.
Pilkington would not comment, but its share price fell 5p to 100p, indicating the market did not expect a takeover bid.
The share price fall was partly because of a downgrading of the company by Goldman Sachs, which said Pilkington was vulnerable to worsening consumer confidence.
NSG's original 10 per cent shareholding dated from May 2000. The recent share purchases, which cost NSG about Ł130m ($191m), are thought to have come to an end, for the time being at least.
One of the sellers has been UK Active Value, the activitist investment fund, which disposed of about 5m shares from its 4.8 per cent stake. "We have marginally reduced our holding," said Julian Treger, joint head of UK Active.
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FT: Nippon Sheet Glass raises stake in Pilkington
Nippon Sheet Glass, a large Japanese manufacturer, has increased its holding in Pilkington, the UK glass group, from 10 to 21 per cent over the past fortnight, but has ruled out mounting a full bid.