[F19Gladiator]

From what I can conclude from the pictures, only the German insignia were painted over and not the code letters. Also the following supports this as the national insignia were only covered temporarily:

The aircraft was flown in this configuration and with a minimum of fuel load in order to be able to take off from the small airfield at Visborgs Slätt. The destination was the larger field at Roma, under escort of two RSwAF J9 fighters (Seversky EP-1). The German insignia had probably been painted over to avoid being fired at by the Swedish AAA again.

When the Heinkel took off from Roma with destination Germany, the German insignia were again visible as they had beeen restored. Supposedly a temporary washable paint had been used. As the aircraft was to be escorted by RSwAF fighters out to international air space it was considered unlikely that the Swedish AAA would use it as "target practise".The AAA unit commanders on the island had now been informed about the planned transfer flight to Germany.

At this time heavy fighting was still ongoing in Northern Norway, particularly around Narvik and the Swedish armed forces had been placed in highest alert on May 18 after Germany repeatedly had requested to transport war materiel as well as personnel through Sweden to Norway, which was rejected, why any German aircraft passing over Swedish territory was fired at (Anything deemed non-Swedish for what that matters!)
It was also considered important that the aircraft would be properly identified as German when reaching the German coast to avoid any misunderstanding in that end.
At 13.05 local time the Heinkel took off for Germany.

It is important to remember that these events took place in connection with that a Swedish B 3 (Ju 86) which had made an emergency landing in Germany was exchanged together with it's crew, with the Heinkel and crew. This involved an intense diplomatic activity in order to solve the situation.

Sources:
Svensk Flyghistorisk Tidskrift 4/2001. Article by Curt Jacobsson.
Svensk Flyghistorisk Tidskrift 1/2001. Article by Lars Gibson.
The Luftwaffe in Sweden 1939-1945, Bo Widfeldt, Monogram, 1983.