Look at the Trijicon website. Based upon what I learned here, I've ordered the Dual Illuminated RMR sights. I ordered one with the Green triangle aiming point and one with the amber triangle aiming point.

I already own a Trijicon 1x24 Reflex (formerly called ACOG reflex sight) but I find that it sits too high for my liking for mounting on a double. I'd need to mount a picatinney rail or a one piece weaver type base on top of the rib, and then the picatinney or weaver base and ACOG would ride on top of that. It works ok on a Remington pump gun with it's really high comb, but not so well on a double simply because of the height it sit above the bore. Another option would be to get a flat bar of steel, longer than the ACOG, drill holes through the bar so one could tighten the mounting screws from the underside of the bar and up into the mounting holes in the ACOG, then where the bar extends out past the front and back of the ACOG, drill holes in the bar and matching drilled and tapped holes in the rib. This way one would eliminate using the Trijicon bases, and thus would eliminate some unnecessary height.

On the Trijicon website they show several bases for various applications. I have ordered the base that fit on the rib "of custom guns" as the Trijicon site refers to it. My Heym 88B PH is set up from the factory to accept this particular base. There are two sizes. The narrow one fits the Heym rib. The wider one fits the width of the Merkel rib.

For the RMR sights I've got on order, I've also ordered an additional base that fits the wider rib of my Merkel 140AE. The Merkel rib will require machining of the oblong hole on top of the rib and the narrow groove on either side of the rib so when I receive the sights and bases, I will need to bring it to a competent gun smith to have the work done. It is an estimated 10 weeks from now before I receive the RMR sights and bases, then it'll be however many more weeks it takes the gunsmith to do the machining work.

Before one goes too far down the road, it would be advisable to get a dial calliper and measure the width of the rib. My modern Heym and modern Merkel have ribs that will work with one or the other RMR base, but I have other older ribbed guns that have odd width ribs that neither base would fit.

The RM57 base is for 11-12 mm ribs

The RM58 is for 14-16mm ribs.