Sheathed smokeboxes - what, how, which, when, why?

[nw-modeling-list at nwhs.org]

Dear All,

Perhaps some of you can answer these questions about the N&W Y 2-8-8-2
locos. They concern "sheathed smokeboxes". I have quite often seen
these mentioned. My questions come in five categories.

WHAT is meant by "sheathed smokebox"? (I presume that it means that
the smokebox has been lagged with some sort of material, just as the
boiler is lagged.)

HOW to determine visually whether a loco has a sheathed smokebox?  I
presume that one of the main ways is to note that it lacks the small
step in the profile between the boiler and the smokebox which exists
in locos with unsheathed smokeboxes. Are there other distinguishing
features?

WHICH locos received sheathed smokeboxes? There seems some
disagreement on this point. In this list, Stephen Rineair commented
that all the Y6bs received the sheathing except #2185. On the other
hand, Kim Thurlow, on his website, at
http://www.ja-gps.com.au/n&w/y6model.html, comments "after about 1954,
all Y6b engines, apart from 2177 and 2188, received sheathing on the
smokebox". Are they talking about the same sort of sheathing? (I
presume that all the Ys were built with unsheathed smokeboxes.)
And Y3/4/5/6/6a? How many of these locos received the sheathing, and
which locos received it? And what about the A 2-6-6-4s?

WHEN did the practice of sheathing smokeboxes start on the N&W? Was
this picked up from other railroads, or was it a N&W innovation?

WHY were the smokeboxes sheathed? Aesthetics? Heat loss? Protection of
workers (from touching very hot surfaces)?

Many thanks to anyone who can enlighten me on these matters.

Frank Hung