Thanks for taking the time to explain and share your knowledge! Additional information on the use is:

1. The single lines sets I want to use are 5 of the Bell System Design line series where there is no possibility of modifying the set for additional off-hook contacts.
2. The sets would be used by no more than one person at a time - this is in a residence.
3. All sets would be located less than 500 feet from the KSU.
4. If the KTU issue makes a difference, I have all the 400 series available (400D, G, & H) to use
5. All sets are home run wired - ideally the A-Lead controller is located at the KSU and the single line sets are wired from there.

Earlier I emailed Mike Sandman about his "HIGH SENSITIVITY LOOP SENSING RELAY" listed on his web site. He replied "The CID2N Modular Series Relay may work. You'd have to try it. $26.95 plus shipping. (The High Sensitivity model was discontinued.)

I wasn't sure if the TT Systems unit had field history on its reliability, since nothing came up when I googled it except that its Yonkers address was up for sale. Sandman's price is half the cost of the TT Systems on eBay.