B767 Maintenance Technician reported they misdiagnosed a problem with crew oxygen system.

Narrative

During an A-check on Aircraft X and carrying out the crew oxy mask/reg inspection task 63-XXXX-X-XXXX. when the mask was removed a leak was observed by the AMT and the inspector. The inspector quite rightly wasn't happy and raised a log page that the captains stowage box was leaking. The issue is that the inspector then in the log page diagnosed what they thought was at fault as the stowage box 'shut off valve'. This was an issue when later it was discovered that the shutoff valve was working and the leak was just within the flow indicator lens; meaning when the mask is stowed both the mask and box don't leak. This made it possible to move the stowage to a different location and lock out that location (2nd observers). Maintenance Control were unhappy that the original diagnoses was made by the inspector in the heading and could cause confusion and worry for the on coming crew.