Saw this mil antenna adapter on eBay

Adapter for NATO standard 4-hole to 2.5 inch antenna mast. eBay item number:127828127664

From the posting:

Racal type 4270-RAC Mounting Bracket, Antenna

CAGE 23657

S/N: none

RACAL PN SS-1600230-4

CONTRACT: F 19628-89-C-0081

Vehicle mount adapter for antennas used on HMMWVs and M939 5-ton trucks for SINCGARS (AN/VRC-87 through AN/VRC-92) as well as AN/VRC-12 Family (AN/VRC-12, AN/VRC-46/47) Also used on the Racal Cougar & Clansman Series.

The bracket's "four-bolt" pattern and heavy-duty studs allow it to interface with several standard military antenna bases, including:

AS-3900/VRC: The standard SINCGARS antenna base.
AB-15/GR: A common side-mount antenna base for older VRC series.
MX-6707/VRC: A heavy-duty base with an internal matching network for broad-spectrum VHF communications.