I have the same lens on a Rolleiflex Automat MXV and it's excellent. My gut feeling with such a low early serial number is it's an Ikoflex 1 850/16 Lps with a Prontor-S shutter. The Compur factory was badly damaged during the war and it was sometime before volume production resumed.
Yes, it says Ikoflex I 850/16 Lps and Prontor-S in the link I provided. The Ikoflex I (a, b and c) continued to use Prontor-S and Prontor-SVS shutters till 1960 to keep the cost down on these cheaper models.
This particular camera may have a flash contact, since it's a late 850/16. Earlier ones looked the same except for the flash contact.
Your Rolleiflex, Ian, is a 3,5 A if it has the stand-alone lever for the M/X sync. At least Franke & Heidecke got the needed Compur-Rapid shutters from F. Deckel for their cameras. Zeiss owned a bigger part of the stocks of the A. Gauthier company than of the F. Deckel ones, so perhaps it was easier for Zeiss Ikon to obtain a larger quantity of Prontor shutters for their early post-war production.