Which initial timing?

My engine number is 7P42453HB which is late 79, early 80 manufacture pre HE. My car handbook and Haynes says 10btc initial timing BUT I’ve seen reference to 5BTC being used on 80/81 pre HE high compression engines. Can anybody tell me the definitive value for this engine number?

Do you have the 10:1 compression pre-H.E.?

Roger Bywater is the guy who was leaning over a screaming V12 in a test cell at Jaguar establishing the advance curves. A better authority is not possible. I once asked him if the timing on the pre-H.E. should be set at an RPM near the torque peak rather than at idle as it is on the H.E., and he agreed that’d be a better idea. This is the result of that discussion:

So, why don’t we adapt the H.E. timing procedure to the pre-H.E.? Roger Bywater: “What you are saying is absolutely correct and setting the timing at low speed before any advance takes effect is just about the worst way of doing it. In all normal operating conditions the advance could in theory be 4 degrees out yet still be within spec. Allowing for wear and tear, not to mention questionable original build quality, the error could easily be more than that.”
Bywater went on to provide at-speed timing specs for all pre-H.E. V12’s:
Carburetted V12’s 35° BTDC @ 4000 RPM
D Jetronic: 33° BTDC @ 4000 RPM
10:1 CR pre-H.E. Digital P 24° BTDC @ 3000 RPM
You just can’t get a better authority than Roger Bywater; he was the guy leaning over a screaming V12 in an engine testing room at Jaguar dialing the advance up and down by hand to develop these advance specs.
Yes, the numbers for the pre-H.E.’s sound like a lot more advance than the H.E.; that’s the nature of the beasts – the H.E. gets less advance from the centrifugal mechanism and more from the vacuum capsule than the pre-H.E.
So here’s the new and improved procedure for timing the pre-H.E. V12: Warm up the engine, disconnect and plug the vacuum advance, rev the engine up a little past the specified RPM and then come back down to it and hold it there, and set the timing to the specified advance.
Of course, one problem is that the scale on the timing plate doesn’t go far enough. So, set the crank at 20° BTDC and paint a new mark on the damper at 0°. Then you can use this new mark to set the timing at the spec minus 20°.