This page gives details of some of the aircraft types that were offered or promised to the pre-WW2 Latvian Air Force but not delivered, cancelled official orders, and types have been falsely reported as being in service.
At least one Albatros C.I is reported to have been used, but the aircraft operated by Latvia were actually B.II examples.
The wreck of one Albatros C.XV (serial 7818/18) was captured in 1919. It was not considered worth repairing, and so did not enter service.
APD AVDI Biplane
One AVDI Biplane was reportedly built by APD in 1929. This appears to be the result of some confusion with the APD AVDI parasol monoplane, with was evaluated by the Air Force in 1928.
One Aviatik C.V serial 12 (ex 8205/18) was reported in use 1920-22. It was actually an Aviatik-built DFW C.V abandoned by the Germans in 1919.
Avro 616 Avian
Bert Hinkler made a non-stop flight to Riga in a Avro 581 Avian in 1927, but the machine continued in his ownership afterwards.
Dornier Do 22
Four aircraft of the Do 22K1 variant were actually ordered by Latvia, but were not delivered before the Soviet invasion in 1940.
An unknown number of P.11s are reported to have flown to Latvia from Poland in September 1939, but none were taken into Latvian Air Force service.
PZL P.46/11 Sum
One example is reported fleeing to Latvia from Poland in September 1939, but none were taken into Latvian Air Force service.
A Polish Salmson 2A2 crash-landed near Nitaure on 15 October 1919, but the wreck was considered too badly damaged to repair for Latvian use.
One example reportedly obtained in 1939, but all military examples were actually operated by the Aizsargu Aviacija (National Guard).
One aircraft was reportedly obtained for evaluation, but the sole prototype actually crashed a few months after its first flight and was not repaired.
MOre background info here http://latvianaviation.com/Purchases_Index.html