Hami5297,
First off, welcome to the Shopsmith Forum.
Below is the information given on the Double Tilt Upgrade online catalog page.
Note: For MARK V 520, 510, 505 / MARK 4 Only -- The MARK V 500 does not have interchangeable table support tubes.
Note: Older MARK V 500 machines (produced before 1972) that have been upgraded to the MARK V 520 or 510 table system will require two new 1-3/4" diameter bench tubes (504163). The old 1-7/8" diameter bench tubes will not fit in the new base castings.
Your Mark 5 serial number 329420 was made in 1956 (a Greenie) and would have the larger 1-7/8" OD Bench Tubes as JPG said. All the Mark 5/V Shopsmith's going back to their introduction in 1954 to today have the same diameter Way Tubes.
As for the first note from the catalog page, the original AKA Mark V 500 Table System has the posts attached differently than the Mark V 510/505, 520, 520S, Mark 7 & 4. Pages 11 & 12 of the Double Tilt Base Upgrade Instructions shows changing the Table Posts. There you can compare the difference in the Table Tie Bar and Posts to those shown in the instructions for the later Mark Models in which the pins holding the posts are easily removed. In the picture below you can see both sides of the Table Tie Bar and posts. In the first picture the pins holding the posts in can be seen indicated by the arrows and in the second picture you can see the pins do not go all the way through the Table Tie Bar as they do in the later Models 510/505, 520, Mark 7 & 4. Also, the pins are located 90 degrees off from the later model Table Assembly.
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Your Table Tie Bar and Posts should look just like this one pictured here above and below. If it does not than something has been changed at some point by either an upgrade or parts replaced.
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The pictures below show the differences of the Tables/Fence Rails between the different Models. Yours should be the 500 which is the original Table. Not all original AKA 500 tables have the extra slot. The extra Table slot was added in the late 1950's (after the Mark 2 was introduced) and discontinued in 1980.
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