I assume you want to buy a threaded 3/8" to 1/4"x20 reducer bushing like this one
General Brand | 3/8" to 1/4"-20 Tripod Reducer | NPTRB
A second accessory you might want to buy is a longer, much larger brass fitting that weighs about an ounce and can be gripped with pliers padded with cloth to prevent scratching to put on very tightly,and which has a hole drilled through the tube to wrench it on or off if it needs to be cinched down tight, with the 3/8 female hole in one end,and the 1/4x20 on a small male protrusion on the other end. This type of fitting is called a "spigot". There are several sub-variations, some with both female receptacles threaded to receive accessories that thread into the spigot, others that have a female end and a male end, and some that are both 1/4 inch, others both 3/8", and those that have both 1/4 and 3/8 inch fittings in male/female, male/male, and female/female. These things are like Legos...with the right ones, you can make anything!
To mount monolight flashes, for the light stand on the left you'd need a 1/4x20 female thread receptacle to 5/8 inch spigot adaptor, and the right side light stand requires a 3/8 inch thread to 5/8 inch spigot adapter. Many strobist type acesories, flash shoes both hot and cold, and umbrella mounts work off of the "spigot" mounting system. Spigots are available for almost any conceivable mix of sixes and male/female studs/receptacles.
The above spigot size info is given *assuming* that your lights use the newer, larger 5/8 size fittings,which took root widely about 30 years old. Significantly older lights often require the smaller 3/8 inch fitting.
This place has the needed spigots, and sells them affordably.
FlashZebra.com: Umbrella Adapters, Flash Brackets, Flash Stands, Clamps & Spigots
