I think they date back to 1919/1920 when the originals came out. The originals were valued at 10kap, 25kap, 35kap, and 1Ruble. Not sure if they were made for the philatelic market at the time or for actual local usage. Latvia was still picking up the pieces from WW 1 and becoming a newly independent state so there may have been a local market for them. The forger is assumed to be Siimson & Kull. Here is a link to a side by side comparison
http://stampforgeries.com/forged-stamps-of-latvia/ I picked them up simply because they were forgeries. I've never seen them offered that often. Value wise I guess it's whatever the collector is willing to pay which for Latvian stamps is usually more than the real ones. The real ones have a CV or $2 hinged or $5.75 MNH.