I'm with Biggs. That's most likely a frame brace of some sort, less likely a mounting system for a forward rack or somesuch.
Certainly looks like it's some sort of stiffener that is holding it all together
It's not the only one though
https://www.ebay.co.uk/itm/167037287524?_skw=moulton+bicycle&itmmeta=01JD8QJ7CDRAHH0TQRXXGRCAB0&hash=item26e432bc64:g:t9MAAOSwjPJm9n7g&itmprp=enc:AQAJAAAA8HoV3kP08IDx+KZ9MfhVJKmkRrvHpcoaecGgHB1L0zljVBbenA8XLcswKyGR/A6TORtPJUf/4z2PR9jArY2usUaytKUCV1G2xEore3twdiFxY80J15gkXR+lLRWbFFJWxVh+sbOveO4xHaPYkX4KeQz7GUf9Mzz5SQG4fxsPxUTLUsi/I3sj7IthdcQP0IK+FkjfwgU5gX6YkKqoaaWaBQFllC3APBDZYRER7WqN7DcOJUmGWvitSdjdyf56k1xwG8VNlH/DY8eej0h+QK7mJMzmKShk5qVLWP2umZt2qgvJIgFqPG8TazJM9EXxtcaJcw==|tkp:Bk9SR6D2yJfqZA&LH_ItemCondition=3000|7000
Did you look at the seller's other items?
They're sitting on a mountain of Moulton parts!