WILL written on ordinary paper is valid and can be submitted for registration. The advantage of registration is that courts give more weightage to a WILL that had been registered over an unregistered WILL in case any dispute arises amongst heirs.
However, it is better that WILL is written in Stamp Paper & must be signed & signed in presence of two witnesses for security purposes.
It is not always necessary to get a probate order for a will. If there is no dispute between the legal heirs as to the contents of a will they may choose to forgo a probate. It is therefore not necessary for a registered will to have a probate, though one may be applied for.