Although the radio end has the bayonets, the actual antenna doesn't have anything for them to latch on to, this is from the PRC68 web site. The actual antenna was held in contact with the big old screw on part.
The radio that I dealt with and made an extension cable and mast for, required the use of a BNC "bullet, with the part that connected to the antenna having its bayonet lugs filed off.
Maybe PRCGUY could help with this if he actually has one available.
Thanks
Joel