Stoker Slides, what are they?

Steel plates...they prevent the full load of coal in the tender from sitting on top of the screw. 

What is the advantage of them being removed?

That I couldn't answer.  I am sure Hotwater or Rich will give the answer on that one.  One things for sure...no one had to go pull the plates as the coal load depleted.

The advantage to them being removed is that you don't have to pull them one by one as the coal is depleted.

With the slides in place, there is only a single small opening (approximately 8" x 18") for the coal to reach the stoker screw. As the coal was used, it would be delivered to the stoker screw only from that small opening. If the fireman did not move the slides as the day progressed, you could end up with a big "hole" in the coal pile and nothing getting to the screw, even though there were many tons of coal remaining in the tender.

With the slides removed the opening for coal delivery to the stoker screw runs almost the full length of the coal space in the tender. There is more weight on the stoker screw, which means the stoker engine has to work a little harder. However, with the very limited miles that a typical steam locomotive runs today, the advantages of not having to go back in the tender and pull the slides several times per day outweighs the slight additional wear on the stoker engine and drive mechanism.

In the 765, we have removed all the stoker slides except the front one. As the coal is depleted during the day, a bank of coal remains at the front of the tender, available for the fireman to use the scoop to trim the fire.

PARTS OF DUPLEX LOCOMOTIVE STOKER

1. Conveyor Trough. 2. Conveyor Screw. 3. Angle Ring. 4. Crusher. 5. Operating Head. 6. Driving Engine Cylinder. 7. Reverse Valve. 8. Piston Rod. 9. Transfer Hopper. 10. Left Elevator Casing. 11. Left Elevator Screw. 12. End of Elevator Screw Shaft. 13. Elevator Pawl Shifter. 14. Elevator Pawl Casing. 15. Distributors. 16. Left Distributor Elbow. 17. Right Distributor Elbow. 18. Dividing Rib. 19. Right Elevator Casing. 20. Oil Box. 21. Conveyor Reverse Lever. 22. Conveyor Oil Cups. 23. Rack Housing. 24. Rack. 25. Conveyor Pawl Casing. 26. Conveyor Screw Flexible Connection Sleeve. 27. Conveyor Screw Flexible Connection. 28. Conveyor Slide Support Roller. 29. Conveyor Slide Support. 30. Conveyor End Bearing and Gear Case. 31. Conveyor Screw Gear. 32. Conveyor Screw Driving Gear.

Here is a picture, I am sure the sides prevented the coal from binding up as it fed to the engine.

They might have been removed for the mechanism to be serviced.

Thanks for the great information!   

There is some good information on type B stokers here for those interested.