I should mention as well:
In my search for those "drive adapter plug" / "drive coupler" pieces, I was directed by an old thread here to
Hazet, who also manufactures them:
6413-1 - 3/8" insert square drive $42.90
6413-2 - 3/8" x 1/4" square through plug $10.86
6413-3 - 3/8" x 3/8" square through plug $9.10
6413-4 - 3/8" x 1/2" square through plug $12.20
http://www.themotoringshop.com/hazet.php?curPage=30
TheMotoringShop.com
800-327-1668
Note that they don't make all three sizes in the straight style: the only straight through model they offer is the 6413-3 - 3/8" x 3/8".
They
do, however, have the oddball 1/4" x 3/8" and the 3/8" x 1/2" drive adapters available.
Note they are considerably more expensive than the Heyco (Wiha) models.
Note also they are calling it a "through plug", presumably to further add to the confusion.
When I called on the phone and inquired about them, it sounded like they were not an item that was stocked and they would have to be special ordered from a New York distributor. My continued searching led me to the Heyco (Wiha) models from KC Tool.
======
To keep things in perspective:
Again, that 1/2" x 1/2" Armstrong S150 sold last night for $17.00 including shipping. Unfortunately I was outbid on it.
The last one I purchased on Ebay was a no-name 1/2" x 1/2" that was in pretty decent condition - it set me back about $14.00 including shipping.
I can't find the listing now, but there was a W1109 Walden Worcester (1/2" x 1/2") listed about 6 weeks ago for $29.99 + shipping. I am assuming it was sold, but I can't find it right now.
Bottom line: pretty hard to beat the Heyco (Wiha) deal from KC Tool.