"Letter of Declaration of No-Break in Services (Continues Services)" which uses for Pensioners, (Most commonly used for Govt Employees) for Revision on Pension or validity of services contains many details and declared by the employee, which supposed to be authorized by the Govt Officials and Department Authority.
The very best solution and option you have in your hand is to go for your nearest local Panchayat Bhawan or Court Premises where you can get many “Documentation Service Provider” who having all the details and ready formats like you are looking for here.
Well I have enclosed a sample format for you and hope gonna work for you. But still request you to please go to Doc-Service Provider for this.