Ask and you might receive an answer...

On one of my visits to HA (Len's shop) I asked about the recoil springs... he gave me a handful and said that he sourced them from McMaster Carr McMaster-Carr

Just now upon searching I have not found a similar spring listed on their web page - he may have had them made up and purchased in bulk.

He also used this same spring as a magazine follower front lip assist when converting mags for 7.62x54r.

The ones I have are 10" long by 3/4 inch diameter. I have no idea of the compression specification.

Hope this info helps.