Fitted is used for both the past tense and the past participle.
Examples:
The tailor fitted the suit perfectly.
The new windows have been fitted securely.
2) Fit – Fit – Fit
Standard in American English.
Fit is used for both the past tense and the past participle.
Examples:
The shoes fit her yesterday.
The puzzle pieces have fit together nicely.
Special Note
Both forms are correct in British and American English, but each region has its preference.
In modern usage, fitted is often used when referring to tailoring, installation, or adjustment, while fit is more common when describing size or suitability.
Summary
Fitted → preferred in British English, especially for tailoring or installation.
Fit → dominant in American English, also widely accepted in British English when describing size or suitability.
Both are correct, but fit is the more universal modern choice.