One example of an NTSC Spectrum (as opposed to a Timex machine) has been found (in Chile). As far as is known, it is the same as a normal (PAL) Spectrum with the following differences:

• The CPU is clocked at 3.5275 MHz
• The ULA is a model 6C011E-3 which generates a NTSC frame size and rate
• One frame lasts 0xe700 (59136) tstates, giving a frame rate of 3.5275×10⁶ / 59136 = 59.65 Hz
• 224 tstates per line implies 264 lines per frame.
• The first contended cycle is at 0x22ff (8959). This implies 40 lines of upper border, 192 lines of picture and 32 lines of lower border/retrace.
• The contention pattern is confirmed as being the same 6,5,4,3,2,1,0,0 as on the 48K machine.

Many thanks to Claudio Bernet for running all the tests on his machine. Three photos are available: 1, 2, 3.