Never before in the 153-year history of rugby union's oldest international fixture had a Scotland player scored three tries in a single game.
But Van der Merwe changed all that at Murrayfield, with the Edinburgh flyer's treble helping Scotland overturn an early 10-0 deficit as they enjoyed a fourth successive win over England, something they hadn't achieved in all games since 1972 and last managed in the championship way back in the late 19th Century.
"He was great today," said Townsend of Van der Merwe, who ended the match one short of Stuart Hogg's all-time Scotland record of 27 tries.
"He just saw opportunities and went for it. I'll be winding him up later to say they were just run-ins but a couple of the run-ins were from quite far to be fair, it was really special."
England went ahead through recalled full-back George Furbank's try and a George Ford penalty as they dominated territory and possession in the opening stages.
But Scotland came away with points from all their first three visits to England's 22 to lead 17-13 at half-time.
"We didn't have much ball and we weren't in England's half very often in the first 15 minutes but we unlocked the defence and Duhan was there to support and to finish it," said Townsend.
Scotland's win, well orchestrated by co-captain Finn Russell, who kicked their remaining 15 points, meant they rebounded from an agonising loss to France and put themselves back in Six Nations title contention with two wins from three games following a tense 27-26 success away to Wales in the first round.
Russell, asked how he felt about being involved in the first Scotland team since 1896 to record four consecutive championship wins over England, replied: "I didn't know that until two minutes ago. It's great for us to be the team to achieve that."
For Townsend, retaining the Calcutta Cup was a huge feat in itself given how much wins over England matter to Scotland supporters.
'Massive'
"This is massive," insisted the former Scotland playmaker.
"It's a one-off. We're playing for a trophy today and it's a trophy that is really important to us."
Defeat ended England's hopes of a Grand Slam, with Six Nations champions Ireland, their next opponents at Twickenham on March 9, now the only team capable of a clean sweep after a 31-7 win over Wales in Dublin earlier on Saturday.
Having arguably played too little rugby in narrow wins over Italy and Wales in the opening two rounds, England perhaps tried to play too much against a Scotland side strong on the counter-attack.
"When you make that number of handling errors at this level, it's very difficult to win, especially against a team of Scotland's quality," said England coach Steve Borthwick.
"Ultimately we made it too easy for Scotland to score but they were very clinical. It's a huge lesson for our team as we grow."
But England skipper Jamie George, playing despite the death of his mother last week, said it was one they could learn in time for the visit of Ireland, who look a class above their Six Nations rivals.
"The fact we are back at Twickenham hugely excites me," said the hooker.
"We want to make it a hard place for opposition to come to.
"Ireland are a great team, we know that. But we're going to be a very tough team to beat at Twickenham."
