Here is a table of information other Two Spool owners sent to me...Thank you All!!! I'll continue to update this as new information arrives.

From the information I've been able to find, National made two spools from 1914-1930, so that may help you get an approximate date for your machine if you go by your serial number.

 Two Spool Characteristics, Differences and Serial Numbers

Serial No.	Badge Name	Cover Plate	Face Plate	Can Type	Hook Style	Base Leg Config	Port/Treadle	Treadle Irons
H2268	Eldredge	Pivot	Hort. Guide	1
H2273	Eldredge	Pivot		1			Treadle
H3068	Eldredge	Pivot	Unique	1			Treadle	Eldredge
H4881
H5862	Eldredge	Pivot	Hort. Guide	1	1	Set forward	Portable	N/A
H6359	Eldredge	Pivot	Hort. Guide	1	1	Set forward	Treadle	N/A
H7718	Blank Badge	Pivot	Hort. Guide	1	1	Set forward	Electric	head only
H8293	Eldredge	Pivot	Hort. Guide	1
H9150	National Odd Badge	Pivot				Set back	Electric
H10474	Eldredge	Pivot		3***			Treadle	Eldredge
H10859	Western Electric**	Pivot	Hort. Guide				Portable
H16348	Eldredge	Pivot		2			Treadle
H18934	Eldredge	Pivot	Hort. Guide	2	2	Set back	Treadle	NSMCO
H22697	Western Electric**	Pivot	Hort. Guide	2	2	Set back	Portable
H25714	Eldredge			3
H28230	Eldredge	Slide	Hort. Guide	3
H29895	National	Slide	Hort. Guide	3	2	Set back	Treadle
H30681	Eldredge			3
H33627	Western Electric**
H36182	Eldredge			1 replaced
H37757	Eldredge	Slide		2
H43535	National			3
H49387	National	Slide				Set back	Portable
H52469	National			3
H53848	National	Slide	45* Guide	3	3	Set back	Portable	N/A
H54778	National
H56444	Graybar*			3
H57095			45* Guide	3		Set back	Table/Motor
H57867	National	Slide		3
H60751	National			3
H62429	National
H66255	National	Slide	45* Guide			Set back	Electric
H68109	National	Slide	45* Guide			Set back	Unknown
H69497	National	Slide	45* Guide	3	3	Set back	Treadle	NationalSMCO
H70248	National			3

*Graybar badged two-spool machines were made by National and sold through Montgomery Wards

**Western Electric badged two-spool machines were made by National

***This can may not work in this machine. Dorothy has not yet sewed with machine. Probably needs a #1 or #2 style
 

Differences I've Found...

A list of the various differences

Spool cans: Three different styles of spool cans (left to right: 1,2 & 3).

Face Plates: Two types of face plates. The smaller thread guide loop is set horizontal, or guide set at a 45* angle and closer to V thread guide.

Cover Plates: Two types; one that is attached and pivots open, one the slides open and off. You can see two differences. The older machine, Eldredge badged H5862, on the left and the newer, National badged H53848, on right. Both plates have the R.K. Hohmann patent info and same patent dates 1911-1913.

Hook Design: I see differences in the hook design associated with the change in spool can design. Left is oldest style (H5862)-uses style #1 spool can, middle (H18934) is a badged Eldredge-uses style #2 spool can, right is the newest (H53848) badged National-uses style #3 spool can. Arrows point to the differences in each.

Spool Winder Arm: Subtle changes in the winder arm. Left is the older style on H5862 & H18934. Right is on the newer H53848-the extra lever assembly functions as a release lever that holds the spool wider up while also holding the rubber motor wheel off the hand wheel when winding a spool (this machine came to me as an electric portable with a motor, which I took off to put it in a treadle cabinet).

Base Leg Position: Style on the right (H5862) has the front left-side leg positioned near the front edge of the base (does not fit into the later style National treadle cabinet) and the other on the right (H18934)style has the front left-side leg back about an 2 inches from the front edge of the base.

Hand Wheel: Original-treadle non-electrified machine head has a heavier and slightly larger wheel (149mm outside wheel diameter, 16mm thick outer wheel ring) than electrified portable machines (145mm outside wheel diameter, 14.5mm thick outer wheel ring).

Badge Decal Name: Eldredge, National, Western Electric or Graybar (a badged Montgomery Wards brand)

Treadle Irons: Some with Eldredge, National SMCO, plain on side of irons.

Spool Cans--The different types

From left to right in all 3 photos; Eldredge decaled H5862 serial#, Worn off decal H18934# serial, National decal H53848 serial#