I am trying to figure out the meaning of the letters on bottom of cups.
AL mark       W.T.& Co. AL U.S.A.         W.T. Co. AL U.S.A.         W.T. Co. AL U.S.A.         W.T. Co. AL U.S.A.
Two different time periods for same AL mark.
{eliminates used as dating mark}
All have nothing in common. {eliminates used as mold mark}
Question still remains, why are the different letters used on the bottom of the dose cups ?  
Conclusion:
The only thing relevant in  the markings are for dating period only  W.T. & Co. 1857-1901  W.T. Co. After 1901
Tarif Act 1891
Whitall Tatum & Company (glass factory in Millville, NJ) was
the most prolific manufacturer of lettered plate embossed
dose glasses
W.T. & Co.
1857-1901
W.T. Co.
1901 after
Fact:
In 1905 Whitall Tatum Company had
opened a branch office in Sydney,
Australia.
By 1916, they had a sales office in
Buenos Aires, Argentina.
Plant #1
Machine #
Below.
A
B
C
D
E
F
G
H
I  
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Plant #2
Machine #
Below.
AA
AB
AC
AD
AE
AF
AG
AH
AI
AJ
AK
AL
AM
AN
AO
AP
AQ
AR
AS
AT
AU
AV
AW
AX
AY
AZ
Fact:
1857 the name was changed to "Whitall Tatum & Company", and finally in 1901, to
"Whitall Tatum Company" which was the name used until 1938 when the company was
sold.
Fact:
There were two locations
used by Whitall Tatum, first the original site (upper works) in
Millville proper, and later the "lower works" in South Millville (formerly known as
Schetterville). Eventually the South Millville location became the center of activity for the
glassworks.
L.L.Samaniego & CIA
{mortar and pestle}
{LLS&CIA monogram}
Botica Mexicana
Ciudad Juarez, Chih. Mex.
{W.T.& CO. Q small}
Note:
The alphabetical numbers on the bottom have no bearing on
scarcity, I have not been able reason why they were used,
my best guess is each plant had a machine to match the
numbers to show which plant and machine made the dose
cup. The W.T. & Co. Marks are the only way to date the
period of manufacture either before or after 1901.
<-------
<------
My conclusion is the numbers are for quality control only.
I also believe cups with no markings or single numbers or letters were made by other Glass Companys.
Dose Cup Dating Page #1
Dose Cup Dating Page #2
Back
I believe it all started here
Tarif Act
1891
W.T. & Co.
1857-1901
W.T. Co.
after 1901
1891-1901
W.T. & Co. U.S.A.
Letter only
before 1891
not W.T & Co.