Why wont they tell you that it holds 10 .38 specials in the manual?
Note what they do tell you, on page 1 of the manual:
http://www.marlinfirearms.com/pdfs/manuals/MFC_Centerfire_1894.pdf
If your rifle is chambered for 357 Magnum,
it is designed to handle 357 Magnum factory-
loaded ammunition (except wad cutters
and shot shells). It will also function
with 38 Special cartridges that are within
the length parameters shown below.
(NOTE: Since 38 Special cartridges are
shorter than 357 Magnums, their use can
cause a lead buildup in the forward part of
the chamber. Be sure to thoroughly clean
the chamber with a wire brush after using
38 Specials.)
Minimum overall cartridge length — 1.400"
Maximum overall cartridge length — 1.590"
.38 Special cases are 1/8 of an inch shorter than .357 Magnum cases, but differences in bullet length and how the bullet is seated in the case means the overall cartridge length differs from one brand/type of ammo to the next. Some .38 Special rounds are less then the 1.400” minimum; you can definitely fit 10 of these in an 1894C with a 18.5” barrel. I have not found or heard of any .38s that are too long to fit 10 in the magazine, but I suspect there are some, which is why I think the manual lists the mag capacity as 9.
Now the kicker. Fitting 10 rounds in the magazine and getting them the feed are sometimes 2 different things. A search for Marlin 1894 on this forum will show a variety of experiences. Some people have 1894s that feed anything, even under the 1.4” minimum length. A few people can’t get .38s of any type to work, or have to hold the rifle a certain way or work the lever sharply to get it to feed. But most find at least one type of .38 that works with their rifle. Bullet shape seems to play a part in what feeds and what doesn’t, and maybe wear does too (at least according to this site
http://www.marauder.homestead.com/files/Marlin94Fix.html)
(You know it has just occurred to me I haven’t read anything about the 44 Magnum version not feeding with 44 Special ammo. Seems to be just an issue with .357/.38.)