Hello Mirek,
The identity of La-5FN tested at Erprobungsstelle Rechlin has been discussed on Russian Sukhoi.ru forum back in 2004, it was almost certainly no deflection and the aircraft got into German hands in 1944, not 1943:
(I assume you can read Russian - see
http://www.sukhoi.ru/forum/showthread.php?t=29796)
These were presumably aircraft of 111 GIAP - in the beginning of August 1944 - possibly on 10 August - a pair of La-5FNs (pilots Leytenant Mikhail Trofimovich Ignatev and Leytenant Chabrov) took off for an Il-2 escort. During a return from this sortie they flew into an area of intense fire of Flak. Both Lavochkins were hit and their pilots performed belly-landings on the German-held territory, some 15 - 20 km behind the front line. They did not manage to set up their machines on fire to prevent their falling almost intact into enemy hands, but managed to cross the frontline within two days and returned to their regiment. They both reported (naturally) that their machines were fully destroyed as a result of forced-landings. Both were later transferred from 111 GIAP. Ignatev later served with 5 GIAP and survived the war as a HSU with 17 + 7 victories.
This is of course just a supposition, the definitive prove of Rechlin´s La-5FN identity would be the comparison of serial numbers, but I do not think these are available from either side, Russian or German.
Hope this helps a little,
Ota