Take a caliper and measure it around the case at 10 and 4. Don't include the Chrono pushers. Voila. 38.5mm.
To be fair the modern SubC TRF 114270 is 40.5mm using the same method.
@Belligero measured the Daytona and the Sub using a caliper somewhere in Europe, and I confirmed the same results at Wempe in NYC, shocking two SAs who did the measuring for me. Now you know why it wears smaller than 40mm. It IS smaller than 40mm.