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The idea of using the angle method with a Stretch bolt is to reduce the torque to preload variation.
The angle tightening method for con rod bolts wasn't introduced until the 84/89 Workshop manual and until then the standard figures were 50/55Nm, so a 5Nm variation shouldn't be an issue.
If it is a stretch bolt when it yields it will not increase load in a linear manner with angle and this helps to control the preload.
If the bolt is marked 12.9 it is surely an ISO Grade 12.9 as for this to have any other meaning would be too confusing.
I am surprised that a Grade 12.9 bolt would be used as a stretch bolt as bolts of this grade wouldn't normally yield until a preload equivalent to around 100Nm.
The nominal maximum torque that you can apply to an M10 Grade 12.9 fastener would typically be 79Nm so even at 55Nm you would be well within the safe limit for an elastic bolt.
I would be interested to know where the statement about using 15Nm and 90 degrees is published as I have looked in the 84/89 manual and can't find any reference.
I wouldn't worry too much as I would be reasonably confident you will still be well within acceptable variation.
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