Obviously as you're in Europe there may be some legal requirements I'm not aware of, but it sounds like you have the same situation as we do here in Australia - a 206 endorsement covers you for the 206L.
It's a bit like the situation where we can be endorsed for all single engined aeroplanes less than 5700 kg MAUW (not sure if you have a similar class endorsement there) - you can legally fly a type without having done it before, and the onus then falls on the owner or operator to ensure you're not going to mistreat their aircraft, and on you to make sure you're not going to kill yourself or cause damage by not being familiar with the characteristics of the particular type.
Getting back to the Long Ranger, considering that when you get your 206 endorsement it covers you for the L, the instructor should be covering the differences when they do the initial training, and the person getting the endorsement should be seeking the information too.
The main differences I remind myself of each time I get out of a 206 and into a Long Ranger are:
a. Weight & balance - obvious differences there, need to know the limits and how centre of gravity moves as fuel burns. Also there's the 'standard pax loading' system if the particular machine is placarded for it, ie pilot plus one up front, must be at least as many in the rear seat as the middle row. If not, W&B must be calculated.
b. Fuel system - two forward cells plus the main, ejector pumps sending the fuel back - possibility of up to 180 lb fuel unusable with boost pump/ejector pump failure (this may vary depending on the setup, I think the earlier Ls may have had a slightly different tank arrangement). Also 3 drain points rather than 1 on the 206.
c. Modulated start to control TOT rather than opening to the start mark (will vary with type of fuel control unit - Bendix vs CECO).
d. Different engine limits, eg TOT, Tq vs. airspeed for mast bending limit - obviously some flight manual study required for this.
e. Fuel gauge switch on some models to see either fuel in the forward cells or total.
Being a different airframe & engine setup there are numerous small differences in what you see on the preflight inspection, it sits more nose up on the skids, has a longer footprint, feels a bit different to fly, but overall not difficult for someone who's familiar with the Jet Ranger.
I've gone on a bit here, but in summary (for our system):
1. 206 endorsement covers the L, so instructor & student should make sure the differences are highlighted.
2. Your flying orders should show what must be covered in endorsement training, so that will indicate what needs to be emphasised when there are differences between variants of the same type.
3. The above mentioned points are what I've found to be the main things I need to think about when getting out of a Jet Ranger and into a Long Ranger.