Your Lee-Enfield oiler was made between 1888 and 1978.  There are five different model (or Marks) of oilers. It is easy to tell them apart. We'll show you how to do it in three steps.

Name	Made of	Size (inches/mm)	In production
Mk I	Brass	4.75" / 120mm	1889 - 1891
Mk II	Brass	3.75" / 95mm	1891 - 1899
Mk III	Brass	3.75" / 95mm	1899 - 1906
Mk IV	Brass	3.75" / 95mm	1906 - 1950's
Mk V	Non-brass	3.75" / 95mm	1950's - 1978

STEP ONE

	ROUND TOP OR KNOB TOP?	Mark	Top
		Mk I	KNOB
		Mk II	KNOB
		Mk III	ROUND
		Mk IV	ROUND
		Mk V	ROUND

Official description	Adopted	LoC
Bottle, Oil, Mk I	12 August 1889	LoC 5877
Judging from the official records, it appears that the Mk I oiler was in production for about two years before being superseded by the Mk II.  Total rifle production during this time was about 360,000 units.
Rifle, Magazine, Lee-Metford, Bottle, Oil, (Mk II)	20 August 1891	LoC 6761
This oiler was replaced by the Mk III in 1899.  Approximately 1.5 Million rifles (presumably with oilers) were produced during this time.
	All Mk I & Mk II oilers have a knob top. The Mk I oiler is one inch longer. Many oilers have Armourers' marks on the cap and side.

STEP TWO

	ROUND BOTTOM OR FLAT BOTTOM?	Mark	Bottom
		Mk I	ROUND
		Mk II	ROUND
		Mk III	ROUND
		Mk IV	FLAT
		Mk V	FLAT

Official description	Adopted	LoC
Bottle, Oil, Mk III	19 December 1899	LoC 10122
The Mk III oiler has a much larger mouth than it's predecessors, as well as a rounded top and a shallow spoon to put oil on the rifle.  It retains the slightly convex (rounded) bottom of the earlier models.
Bottle, Oil, Mk IV	22 March 1906	LoC 13470
Finally, in 1906, someone figures out that a flat-bottomed oiler will stand up where you put it, instead of falling over.  The design will remain unchanged for another forty years.  This is the most common oiler and remained in production at least until the 1950's.
	There is no discernible difference between the Mk III and Mk IV caps and spoons.  There may be slight differences by manufacturer.

Occasionally one encounters steel or recessed caps on Mk IV oilers.  These are WWI economy measures.  The steel caps are unique to WWI WESTERN ELECTRIC oilers; the recessed caps sometimes show a HARCOURT patent mark.
Steel caps are a scarce find as many have succumbed to rust.

STEP THREE

	BRASS OR NON-BRASS?	Mark	Material
		Mk I	BRASS
		Mk II	BRASS
		Mk III	BRASS
		Mk IV	BRASS
		Mk V	NON-BRASS

Official description	Adopted	LoC
Bottle, Oil, Mk V	WWII	Unknown
Approved during WWII as a production economy, the Mk V oiler is generally plastic, bakelight, or steel, often without inspection or easily recognized makers' markings.  There are many variations of spoons.  Generally, the amber or translucent oilers tend to be much later post-war production.

That's it!  Three easy steps and you, too, can impress your family, awe your friends, win bar bets and save Western Civilization with your knowledge of Enfield oilers.