No. 8 - 1891 - 1910
38 L.I.L. (38 Long Inside Lubricated) Colt
Showing cavity former that could be reversed for a solid-base bullet, but why?
Scarce 32 L.I.L. (32 Long Colt)
Rare British (Pre-1902 .450 Adams), seen on eBay.  Note rounded cavity former.
Early (Pre-1902) 38 L[ong]
41 L  C D A
(Long Colt Double Action -
Pre-1902 seen on eBay)
In response to the above question, I received this from an ARTCA member:
"I have a conjectural answer, which I am not sure I believe.  Note that the
mould in this example has the ca. 1902 improvements (sprue-palte pivot
screw lock screw and alignment peg and matching recess in the mould faces).  
By that date, Colt had (perhaps following S & W's lead with the longer and
more potent Black Powder ".38 Military", which became the smokeless .38
Special) had reduced the groove diameter of their .38 Long Colt revolvers
from the original 0.376" (roughly, the OD of the case for the outside
lubricated, heel-crimped bullet loadings of the early cartridge era) to the
inside case diameter - in their case 0.355" (while the S&W  went to 0.357"
-0.358"); and requiring an expandable base for use in the older revolvers.  
The reversed base plug could be used to cast a slightly heavier plain base
slug fitting the new, smaller groove-diameter barrels at the
dawn of the 20th Century.
Unfortunately, like some good-at-first-glance hypotheses, this one falls apart
on closer scrutiny.   First -and most obviously - the pointed "pusher" for the
bullet sizing orifice - designed for the near-conical recess in the standard
bullet base - would be almost impossible to properly center on a plain-base
bullet, virtually guaranteeing poor sizing results.  
But if one takes your hint of reversing the loose but captive base former,
flat-side up (most such formers for shallow hollow bases, in Ideal, Marlin
and even early Lyman practice, have a slightly domed lower end which
protrudes slightly below the base of the mould; reversed, these would give
a shallow, cupped base; the dead-flat bases on these tools are somewhat of
an anomaly), one gets the result:
that the new, flat base intrudes on the lower of the two grease grooves,
giving a significantly shorter, lighter bullet,with an oddly-rounded
"bevel base", 'way before the time when the latter were developed for rapid,
production loading."
I don't know where I got the idea that the intended use for the cavity-former
had a dual purpose as I haven't seen any mention of it in any of the Ideal
catalogs and have yet to run across any mention of a reloader doing that in
any of the old magazines.  Nevertheless, those old gun cranks did push the
envelope, so if anyone runs across a mention along this line, I'd been happy
to hear it - and post it here.
Some other calibers in this tool.
Casts and loads a hollow-base inside lubricated bullet
for cartridges that were initially issued as outside lubricated.
The present owner believes this to be a No. 8 with the mould ground off, as so often shows up.
But . . . the No. 8, as far as is known, was never issued with an adjustable die.
On the other hand, the few No. 3s seen in this caliber have the standard "blunt" bullet-sizing plunger,
with the tapered point version seen only in the No. 8.
Note the combinations of very early die ID number: "40-70", with the "40" being Ideal's reference number
for the caliber and "70" being the bullet design (35870) and "38 L.I.L."
Photos courtesy D. Elliott
I believe this recent photo proves to me, anyway, that the tool is a No. 3 with the tapered point bullet-sizing stem.
Nice Find!
I always wondered if Marlin made a No. 8, so
now another one of Life's Mysteries is solved
- and in a scarce caliber to boot!