Yep, .38 rimfire ammo hasn't been made since about World War II or slightly before. Any vintage ammo you find today may not fire due to the degredation of the priming mixture. Or it may fire. Who knows?
But I wouldn't fire the stuff. .38 Rimfire is pricey, and getting harder to find as the years pass. It was never a rare cartridge, but I've noticed less and less of it at gun shows as the years go by.
That firm in Britain seems to have a good alternative for making rimfire cartridges. You have a nice, old Colt.
I reload heeled bullets in my Marlin Model 1892 lever-action rifle in .32 Long Colt caliber. I also have a firing pin to shoot .32 rimfire cartridges. I have experience with casting and reloading heeled bullets.
To reload for it will require heeled bullets with a heel of probably around .358 and a bullet diameter of .375 inch.
Gad Custom Cartrridge sells these heeled bullets. You may also find that conical bullets for the .36-caliber cap and ball work too, since the .38 rimfire initially used a bullet nearly identical to the 36 conical.
Do NOT use smokeless powder for reloading. Your revolver is not strong enough for the sudden jolt of pressure that smokeless powder delivers. Black powder delivers a gentler pressure curve.
Use FFFG black powder, filled to within about 1/16th of the case mouth. Seat the heel of the bullet firmly onto the powder so that you get a slight, satisfying crunch. This will indicate that the powder is slightly compressed, a condition under which black powder burns best.
You may also use Pyrodex P black powder substitute but do not use Hodgdon 777. The 777 is more energetic than black powder, is not an across-the-board substitute for black powder. I would avoid it because it can produce slightly higher pressures than black powder.
After seating the heeled bullet, turn the entire cartridge upside down and dip the bullet in melted black powder lubricant such as SPG or Lyman Black Gold.
With the bullet in the lubricant, right up to where the bullet meets the case, you'll notice a ring of surface tension around the bullet. Remove the bullet at this point and you'll get a big gob of lubricant it. No problem, though, simply return it to the melted lubricant and hold the bullet there.
You'll get the same surface tension ring as the colder lubricant and bullet warm to the temperature of the melted lubricant. Suddenly, the tension ring will disappear. This means that the bullet has reached the temperature of the melted lubricant and it's time to remove the cartridge. At this point, with no surface tension ring, you'll get a nice, even layer of lubricant on the bullet.
Stand the loaded, lubricated cartridge upright on waxed paper and let the lubricant cure overnight. Store the loaded ammo in plastic, flip-top ammo boxes. Buy two boxes: one to store ammo in, and the other to put the greasy, sooty fired cases in. This will avoid cross-contamination.
When the gunky ammo box gets too nasty, simply put it upside down in the top rack of the dishwasher.
You'll need to clean your revolver and fired cases with warm or hot soapy water after firing, to dissolve the black powder fouling which is almost completely water soluble. Yes, you can buy commercial black powder solvents but soapy water works as well.
While cleaning, carefully avoid trickling water into the action. You may have to remove the grips to ensure no water got inside, but don't disassemble it any further. Dry the revolver, without grips, in the hot sun or on a pan in the oven at its lowest setting. Crack open the oven door to alllow moisture to escape.
Remove when warm and re-oil. I prefer to oil my black powder revolvers with canola or olive oil, though lately I've been told that olive oil is slightly acidic and canola is preferred.
I would avoid using WD-40 as it becomes a hardened varnish over time and gums up the works. I know this from experience.
Take care of that old Colt and will last for decades. Firing it is kind of a pain in the patoot, what with making cartridges and the cleaning and all. Some folks would urge you to NOT fire it. But if a gunsmith has looked it over and pronounced it good, you may safely fire it with black powder and heeled bullets.
If you do fire it, let us know how the ol' girl did. We'd be interested in accuracy, function and muzzle velocity from such an old piece. Take good notes at the range.
I've never read of anyone loading and firing a .38 rimfire, let alone testing it for accuracy and velocity. You may be the first, if you choose to do so.