Dan,
Good question about Saw Guards. The Mark V 510/505. 520, 520S, Mark 7 and Mark 4 all use the same Saw Guard and it is different from the Saw Guard versions made for the Mark 5/ V 500 original Table System. They are not interchangeable. For the Mark 5/V 500 I will start with the most recent Saw Guard version since that is what I believe you have from your description. It is part number 505875 and it's first version is shown below.
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The first version of this Saw Guard had a single 1-1/4" Dust Chute. Not very efficient. An upgrade was made to provide two 2-1/2" Dust Chutes as seen on the following picture of the revised version.
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Below is the upgrade parts to convert the earlier version.
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The Upper Guard attached to the Table Post/Tie Bar which is the only Mark 5/V 500 Saw Guard that does that. All other Mark 5/V Saw Guards, including the one for the other Mark V models, have the Upper Guard attach directly to the Lower Guard. The one disadvantage of the 505875 Saw Guard Assembly is that the upper Guard attaches to the Table instead of the Lower Guard so it cannot be used for the extra Table Slot your Mark 5 has.
Going back to the next earlier Saw Guard is the all metal one, part numbers 505627 (upper) and 505628 (lower) shown below.
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It attaches only to the Quill. There was a different but very similar lower Saw Guard part number 505705 made for the 1960's Mark VII that uses the 505627 Upper Guard.
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That brings us to the first Mark 5 Saw Guard part number 22 370. The part number is much different from the others since it was made prior to 1964 when all part numbers changed to the six-digit numbers we more commonly see.
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At first, the Saw Guards were an optional accessory. These Guards listed above do not include the Saw Guards for the Mark 2 or the Model 10's.
Lastly there is another lower Guard (P/N 555167) for the Mark 5/V 500 which is a stripped-down version of the lower Saw Guard for the Mark V 510/505. 520, 520S, Mark 7 which is used for dust collection when using the 12" Sanding Disk.
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