Option 961 Special Engine

Sure Reinhard would give us a bullet proof explanation, but yet I give it a try:
as far as I remember, the "Option Special Engine" means for M60 and early M62 V8 an Alusil cylinder block, where these were usually of Nikasil alloy.
Later on (probably during 1996 and on) all M62 were built using solely Alusil.

That would have been my best guess

S961 started in December 1995 and covered the remainder of 11 M60 for the UK. It continued throughout the M62 production until January 1997 (total cars with S961: 343) and apart from 6 cars for Latin America and 1 for Germany solely applies to cars manufactured for the UK. I´d assume that the same type of engine was actually also used for the US shipments only not specced in the build data sheet under "order options".

Cheers
Reinhard

The Alusil M62 was specified for a number of markets including the US and the UK:

Crankcase
For production reasons (production capacity at crankcase manufacturer), two different aluminium alloys are used in the production
of the crankcase.
The crankcases of engines for markets with critical fuel quality (high sulphur content in fuel) are made of Alusil (same as M73).
These are engines for
λ USA/CND
λ GB (incl. Ireland)
λ South America
λ Mexico
λ Saudi Arabia
λ South Africa
In the same way as the M73 engine, the crankcase of these engines is made from an aluminium alloy (Alusil) in a chilled casting
process. The cylinder barrels are not coated. The required surface quality of the cylinder barrels is achieved by means of an
etching process as part of the production process.
The crankcases of the M62 engines for all other markets are made of the aluminium alloy (AlSi9Cu3) as known from the M60
engine. The cylinder barrels of these crankcases are coated in a nickel-dispersion process (Nikasil)
The part number serves as the external identification feature for both crankcase variants (aluminium alloys). The two material
variants of the crankcases also feature correspondingly different surface coatings for the pistons (refer to 'Pistons').
Irrespective of the different production methods, the design of the crankcases is identical for both variants.