Basically since a builder is required to build at least 51% of an amateur
built aircraft he/she becomes the manufacturer.

From:
FAR 45
§ 45.13 Identification data.
(a) The identification required by § 45.11(a) and (b) shall include the
following information:
(1) Builder's name.


From:
AC 45-2A - IDENTIFICATION AND REGISTRATION MARKING

5. IDENTIFICATION OF AIRCRAFT AND RELATED PRODUCTS.
d. Section 45.13(a)(1), Aircraft Builder.
   The "builder's name" on the identification plate may be an individual,
firm, co-partnership, corporation, company, association, or joint stock
association, and would include an amateur builder or aircraft manufacturer.
For an aircraft built from spare and surplus parts, the builder's name would
be that of the person who assembled the aircraft and not the name of the
manufacturer who builds the same model of aircraft.


From:
Order 8130.2D - Airworthiness Certification of Aircraft and Related Products

b) Aircraft Builder's Name (Make). Enter the name of the builder or
manufacturer as it appears on the aircraft identification plate (§
45.13(a)(1)).
1. For amateur-built aircraft, the aircraft make is the name of the builder.
When two or more persons are involved, enter only the name of the individual
that is listed first on the aircraft's identification plate.
2. For aircraft built from spare and surplus parts, the aircraft make is
that of the TC holder (as it appears on the applicable aircraft listing,
specification, or TCDS) together with the name of builder, e.g.,
Bell-Jackson.
3. For surplus military aircraft (not assembled from spare and surplus
parts), the builder's name will be as listed on the aircraft identification
plate.


Hope this helps.

Ted Stanley - A&P-IA