3X plug; original?

[Brubaker]

Is the extent of the knurling(to the bottom of the base) the deciding factor in determining if a plug is original? Three-out-of-four are original, but one is in question. The '3' is rounded, but maybe there were some early repro's that style(?).
One one the left I'm sure is original, one on right questionable.
Thanks as always.

 

[peters180a/170b]

The one on the rIght came out in 1932 and is for the Model "A" too! [a later plug] not repro] . I have a set of n.o.s. ones and on the box [ For "A" and "B"]. You can see the difference not only with the base [which] you can change but the give away is where the "3" is under the letter "M" [it is spaced different from the orginial one on the left - which is correct.

 

[Brubaker]

Thanks Peter!

 

[FL&WVMIKE]

BRUBAKER & peters180a/170b ....................
I have three strange 3-X plugs. They are marked,"FORD A". I assume that these are an early replacement plug. Does anyone have a theory ?
MIKE

 

[FL&WVMIKE]

1930-1931 Spark Plugs Waiting For Restoration.
Note the knurled base is 1/4" tall.
MIKE

 

[FL&WVMIKE]

1928-1929 Spark Plugs Waiting For Restoration.
Note that the knurled base is 3/16" tall. The top center plug is some strange mixture.
MIKE

 

[peters180a/170b]

I have seen then before [ford A plugs] but don't know. Maybe it was a core replacement??