There were two typical adapters to fit the 3/8" bar stud slot bars to saws with the 5/16" studs.
One was a small "spring" the fit over the 5/16" studs with a 3/8" O.D., you could likely make the same kind of adapters by cutting short sections off a piece of tubing with a 5/16" I.D. and 3/8" O.D. Some of the bars being sold on e-bay today (GM in OH?) note that they will work with either large or small frame saws and come with the adapters.
The second was an "S" shaped sheet metal that fits over the 5/16" bar studs to space it out to 3/8". Again you could make your own from a thin strip of 22 gauge material.
I have both types at home but don't remember the part numbers off the top of my head.
Bars with the 3/8" slot will fit everything from the D-44 all the way up through the SP125 including D's, 55's, 1-4X, 1-5X, 1-6X, 1-7X, 1-8X, 200, 300, 400, 500, the old gear drive 600, 700, 800, CP125 and SP125. Include the 33/35/39 saws as well as the BP-1.
The only possible catch is some of the D-44 and 1-40 saws used a bar adjust scre withi an oversize head that fit a milled notch in the bar rather than the more conventional adjuster dog and screw arrangement.
Mark