What are the differences between, “morgens”, “am Morgen” und “XXXmorgen(s)”?

Question

What is the difference between the meanings of the following sentences?

1) Wir haben uns Montagmorgen getroffen.

2) Wir haben uns montagmorgens getroffen.

3) Wir haben uns Montag morgens getroffen.

4) Wir haben uns montags morgens getroffen.

5) Wir haben uns Montag am Morgen getroffen.

6) Wir haben uns montags am Morgen getroffen.

All these sentences are supposed to express what happened on monday morning. Not to be confused with "Montag morgen" or "montags morgen" which doesn't exist, since the lower case "morgen" means tomorrow. My feeling says that 2, 3, 4 and 6 could have happened more than one time, whereas 1 and 5 just happened once. Are there other differences?

Answer 1

1) means you met each other on one particular Monday morning.

2) means you met each other regularly on Monday morning.

3) is just the old and now incorrect way of writing 2) before the German spelling reform.

4) is almost the same as 2), but it separates the day and the time: you met regularly on Monday, and these appointments were normally in the morning.

5) is another and not so idiomatic version of 1). You emphasize that the encounter was in the morning (and not in the afternoon).

6) Analogue to 5). You met regularly on Monday morning, and you emphasize the time.

Answer 2

I think 1) means "we met on Monday morning", i.e. "in the morning last Monday", while 2) means "we used to meet on Mondays in the morning". As for the rest, I think 1=3=5, while 2=4=6.