As of 2016 TracTronics SECSI is no longer online. It previously appeared at www.tractronics.com . A archived version of their website is available on the Internet Archive.
SECSI support will remain in JMRI, but it's unlikely to be updated any further if there's no active manufacturer of components.
The system letter is "V", so for example the first Sensor on node 5 would be "VS5001".