Here's a nice old lens board for an Eastman Improved #2 that uses the 5 3/4" one. I've had several (11X14 and 6.5X8.5) that needed this size.
Center cut out is 2 3/4". Hole drilled for a packard hole. $8.00
Not-so-pretty Packard shutter, but it seems to work spiffyl Aperture is 2 1/4" Shutter only, no hoses or fittings.
This one works spiffy. It's a #5 which does not have instant. Just open and close. $40 U-pay-shipping please.