To add to what Ben said,
The 3/4 size couplers we purchased from EBT hoppers that were (and still are, presumably) destined for scrapping/parting out were not as suitable for use as we had hoped. We'll still find a use for them, but our efforts to secure a reliable source of 3/4 size couplers has taken a different direction. I'll let others more knowledgeable fill in the details, if they are willing/able to do so...
A reliable, cost-effective source of 20" wheels remain an issue. We are working on it, but they are pricey.