[R-390] Question - Dave Curry Longwave Products Mechanical Filters - Still Available?

[WA3FRP at aol.com]

I recently found out about the Curry Longwave Products Mechanical  Filters.  
I'd like to find out if the R390-A 500 Hz mechanical filter is  still 
available but the links I've been able to find point me to the ER Store  which seems 
to be non-functional.  I'm using my R-390A every day on the ham  bands for RTTY 
and the 500 Hz filter would be great during contests and when the  bands are 
crowded.
 
Can anyone help with a purchase source or provide a filter?
 
Thanks!
 
Russ
WA3FRP