I gave Colin some of that information, and I think I can help:
3 boats were contracted in early 1925 and delivered to WCC for the 1925 season: 939, 940, and 941.
A 4th boat, 942, was delivered to Halsey DeWolf (I assume of Bristol) in 1925 and was not part of the WCC fleet. A 6th boat, 943, was also delivered in 1925, but most likely not to WCC.
7 boats were contracted later in 1925 and at least 5 were delivered to WCC for the 1926 season: 972, 973, 974, 975, and 976. The other 2, 977 and 978, were delivered in 1926 but not specified in the record as going to WCC. That makes a total of 10 possible boats going to WCC, but at least 8. Rick Meyers talked about 9 WCC boats in his document, which is where that figure came from. Let's go with 12 (10 possible WCC boats plus 942 and 943).
Since Colin's boat has a marconi rig and moulded sheerstrake, it could be one of these 12, plus the last 2 boats built, 1379 and 1380. However, since Colin's boat has an oak sheerstrake and the last 2 had mahogany, BUTTERFLY is almost certainly one of the 12 contracted in 1925 and delivered in either 1925 or 1926.
Of those 12, 941 and 976 have been accounted for. That leaves as possible candidates the following 10 boats: 939, 940, 942, 943, 972, 973, 974, 975, 977, or 978.