Described as a pale blue silk blend, full skirted summer dress, the custom creation was based on a dress in the current spring/summer collection from the New Zealand-born and London-based designer.
"It was made specially for the duchess," says a spokeswoman. "We've had a style in store that's very, very similar. So it was very slightly bespoke for her."
The original is named the CZ dress, and it comes from a collection inspired by Truman Capote's "society swans" – one of whom was CZ Guest. Although now sold out, it originally sold for £990 (around $1,500), but the bespoke version costs an extra $300, so Kate would have spent around $1,800.
Royal chronicler Judy Wade was at the party at the National Portrait gallery and believes it to be Kate's "first public maternity dress."
"Even though it's not designed directly for pregnant women," she says, "it is easy to adapt because it is an empire line design."
Wade adds: "At this stage of her pregnancy an empire line dress looks great and fits perfectly. She looked pretty and the sleeveless design was right for a room full of people so she stayed fresh and cool all evening."
The spokeswoman meanwhile also tells PEOPLE that because of the so-called Kate Effect, the company says they have had a "few orders placed" for new bespoke versions since the charity event.
Ever the fashion recycler, Kate paired the dress with some gray suede "Malone" heels by Rupert Sanderson, which she has worn before. The £375 ($570) shoes were originally seen on an outing to Fortnum and Mason, London with Queen Elizabeth and Camilla, the Duchess of Cornwall, in March last year, as well as the previous summer at Windsor Castle when husband Prince William was at the Order of the Garter service.
A spokesperson at Rupert Sanderson says, "They are a quintessential shoe in the sense that they are reflective of Sandersons 'less is more' design ethos."