All of the Stromberg carburetors have an o-ring seal for the base. The Brass base on the right is the earlier style. It actually screws into the carburetor. The o-ring is not pictured in either of the bases for clarity. It goes into the groove. It is important that the o-rings are of the correct size and material. Get either of these wrong and you will end up with a fuel leak. On the Jaguar XKE, the brass style was used on the 6 cylinder cars. The factory switched to the plastic style on the left early in the V12 production. Both had brass bases so might appear the same at a casual glance.To remove them is very straight forward. The earlier brass style must be unscrewed. If access allows, a large screwdriver works. In tight spaces, pliers might have to be used. They are usually very tight from corrosion. If you look at the one on the right, you can see where pliers have been used to turn it. The XKE is rather tight under the carbs and removing the carbs might be necessary. The V12 on the other hand can be changed on the car. Access to them will be easier if the air filters are removed first. To start removing them, take a screwdriver and start to pry the cap off. Then you can grab it with your fingers and wiggle it off. In either case, there will be some fuel spillage so have a rag under the carb to catch it. The aluminum float bowl has a slight taper on the inside of the hole so the plastic holder will come out. With age, plastic gets brittle so if more than one tab breaks off, it is a good idea to replace the o-ring housing.
Remove the o-rings and clean the base piece. Install the o-rings and put a little oil or grease on them to ease reassembly. The later plastic ones just snap into place. The screw style have to be screwed in. A little grease on the threads will help here also. After they are installed, turn on the key to let the pump run and check for leaks.
Congratulations, you are done. This procedure will probably need to be repeated every 10 years or so. The easiest way to tell is to touch the bottom of the carb and see if it is wet. The fuel should be below the gasket so if it is wet, the o-ring is the best suspect. The XJ6 Series cars also used the later plastic style bases and are easier to access on the car. These are available for sale on this site. Click HERE to go to the order page.