Ncorry,
I went down to my game/gun room and double checked the filter, it is a 37mm Rocolax ND8. You can order one directly from Rocolax, and Google will find their site. Delivery takes awhile since they ship direct from Taiwan. I would recommend a 35 or 36mm filter if they made one, but 37mm is the smallest they offer. It is a tight finger press fit, being careful as you work it in. A little lube like dish soap might be a good idea.
Fwiw, they also offer PVS 14 IR filters to protect a PVS 14 against excessive light and mistakes like leaving it on in the truck and they work as sacrificial lenses too. I use 720nm filters on my PVS 14s when the USGI 3x isn't screwed on. I think they're 31mm, but I would have to check to be sure of that. They do cut down performance, and take the performance of my high 28s SNR tubes to about the same as an unfiltered 24-25 SNR tube. Small price to pay for the piece of mind though, and on very dark nights I will take them off to get the best performance.
I looked at the BC and no size is given. Down on the table where I did the work there was a BC package which shows size "10 OBJ" and I'm pretty sure that is the size. A 10 OBJ red lever release style BC cap that was sitting there too. I think I got both and used the one that fit better. And I have noticed that the lever version and the non lever versions often have slightly different fit on stuff. The cap will not stay in place without some adhesive and I used BoatLife marine adhesive and it has held up for a couple of months and maybe six or seven night hunts or deer hunts. I had to remove the Pulsar rubber eye "cup," which ain't much.
I definitely prefer the dimmer view through the filter and if I got another 19A or Pulsar spotter I couldn't dim enough through the controls I would do this improvement in a heartbeat. When I first used the set up I fogged the filter a couple of times, but if you fog it a couple of waves through the air clears the fog, which is only on the outside of the filter.
Since I wanted to keep the option of helmet mounting the 19A I glued the cap to open up, knowing it would mildly interfere with a ball cap, and it does. If helmet mounting doesn't interest you setting the cap to open to the side would be better. Right side if you scan with your right eye, left side if you scan with your left eye. I didn't try down but think it might hit my cheek.
Hope this helps,
JPK