SB 13 Sheet Metal Question

[Gearbusters]

On or about what year (or Decade) was the 2 Piece Sheet Metal Gear Guard introduced on the SB13 ?
See the Attachment

Gearbusters

 

[GenePoole]

A rough guess: 1950's

 

[Gearbusters]

That was my guess too... What about the arrival of the Lever Style Clutch vs the Star Wheel ?
When did the Lever Style Show Up ?
I have bought a 13 Big Bore with a Lever Clutch. S/N and Cat No are both painted over and I have not picked up the lathe yet...
There are 2 bolts where the s/n should be - no clue what that's all about. The s/n's are always cut into the Bed (?)

JP @ Gearbusters.