The Black Powder Variant of the 55 has some internal parts that are brass and it is easier to disassemble for cleaning. The residue of Black Powder and some of the substitutes is very water absorbent and will draw water right out of what you thought was dry air!
I used a Hornady smokeless powder drop for one of the subs and didn't take it apart for three days. It was totally corroded so an otherwise good measure in the trash.
I like the Orange 55, if the price is right I would buy it as a smokeless measure. It is easy for me to spend other peoples money but DO NOT USE A SMOKELESS MEASURE FOR BLACK POWDER. An affordable substitute is a muzzle loader measure, but make your adjustments by actual weight not by volume. I don't use the flaky two piece drop tube that came with the 55 BP, but I use a 2 foot long brass drop tube from Shiloh Sharpes to fill my 45-70 and 45-90 cases. It will settle/compress a 65 grain charge by a half inch or a little more, the key is a consistent slow pour (I count to 4 or 5) not a dump. The tube has a funnel at the top, pour a little off center too.
Hints:
Be sure to include a lube cookie or Ox-yoke wad over the powder. Most loading instructions omit this, but it is necessary to keep the fouling soft for accuracy (or swap the barrel after every shot). If using the "cookie" you will need an over the powder card.
Different sizes (FFg FFFg ect) and different brands of black powder will be more consistent with different primers, some prefer large rifle and some prefer large pistol. There is only one way to fine out, keep notes!
A good condition rifle with good black powder ammo will be capable of groups as small as 1.5" at 100 yards. If you get 4" groups you won't win many prizes, but you can ruin a ground hog's day out to 200 maybe 250 yards. (you guys out west have Rock Chucks, they are 2/3 the size) Remember: if you learn something, no shot is ever wasted!
Ivan