Difference in Tubes?

I need tubes for my 29 CCPU. Coker sells a 440/450-21 Tube TR-24 with Offset Rubber Stem for $31.95 each. Snyders sells its rubber-stemmed tube for a 21" wheel for $15.75. Anybody know what is the difference?

And before some wag finds his calculator to tell me the difference is $16.20, I know THAT...:)...the difference in type or quality, I mean, what accounts for one being twice as much as the other?
 
Offset stem will not work on a Model A rim, since the stem is in the middle of the wheel. Offset tubes are for when the stem sticks out the side of the wheel (solid steel wheels)
 
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