I have never owned a later E type Ross with the 16tpi barrel, so I haven't had one apart or seen the underside of the breech. However, your last picture seems to show the locking screw for the left hand thread 3tpi type action. My picture shows the screw backed out to be more easily seen. The later type E with the slower thread would have no purpose for this locking screw but that doesn't mean a blanking screw wasn't there just to cover the hole because they may have used the same receiver blank to make both versions. If you back out this screw you will know for certain as you can then see the barrel threads. In any case it seems you have worked things out to your own satisfaction. Best. Tom