The keyer timing with all the MTR2/3/4/5 is (fortunately to me …) Iambic B (not explicitly mentioned in the manuals, but confirmed by Steve).
The difference between Iambic A and B explained by Chuck Olson here:
http://wb9kzy.com/modeab.pdf
I have read that description before and found it confusing. I think I know why. It does not explain that the extra element sent in mode B is due to having that paddle closed during the sending of the last element of the other paddle.
If the opposite paddle is not engaged at the same time, the extra element would not be produced.
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Heinz,
I agree, the MTR3B uses iambic mode B. The source on eham.net, I had my information from, is wrong.
The example with the letter “c” in the explanation of Chuck Olson is an unfortunate choice. I tried several letters and found that (for me) the difference between iambic mode A and B is most pronounced when trying to send the letter “a”: Being used to iambic mode A, I produce almost always an “r” when in mode B 
73, Roman - DL3TU
Looks pretty slick Fred. Do you have construction details for the EFHW tuner posted somewhere?
I just bought an MTR3 yesterday! I have some long backpacking trips planned for this summer, and the MTR is going to lighten my load quite a bit (compared to the KX2 I usually haul around).
Wow! Fred - your EFHW tuner in the dental floss box made me greedy to make my own. Would you please share more details and pics of the build? Thank you in advance.
PS. If you don’t I will nag you everyday… 
73!