For the most part the filters are all made by Collins (and are no longer in production), so all the options go back to the same basic filter.?
The various retailers mount the filter on a board suitable for your rig.
For me, inrad has been a good source when I couldn't find a used filter on ebay.
I dont think W4RT is still in business (correct me of I'm wrong) and they were the one who made the board that let you use 2 filters on the 817.
Hope that helped.
John - KE7WNB?
