It's funny you should mention this boat because I've just added/updated the records relating to this model just recently. Don't know if you saw them. If we are talking about the same Halsey Herreshoff design, I believe they were sold under the the Sailstar brand before it Became Bristol Yachts. There was the SAILSTAR 26, and the COURIER 26. Both seem to share the same hull but with slight differences in the coachroof and interior. The COURIER seems to be extremely rare. It was probably the SAILSTAR that became the BRISTOL 26.
It may be difficult to find an exact mast height number since the rig dimensions may well have been changed a number of times.
Do you have the orginal rig? Why is it that you need this number specifically?