i made those. however I no longer make them for that size of bulb. it sucked trying to keep some in inventory for each different model, so i standardized it. now you must use a stopper, but those can be had from bomgaars for 90 cents. I charge 25 dollars shipped for a set of arms.
Also, i never made any for the showdown, as they have a different mounting system. there is a guy that makes them for showdown units, but they are powder coated and not engraved.
to mount the arms, you will have to drill a 1/4" hole into the base. i include a riser and hardware with them to mount it.
another note, they are no longer polished. my polishing guy got transferred from the machine shop to the water jet/laser department. so i refined my clamping technique and have improved the machined surface to a very very fine micro finish. another advantage of this is not having to keep them super clean, since the water marks really show up on the polished units.