Self-Play Scenario OBA question

[cyberjockey]

I'm getting ready to play my first scenario for the self-play path which is Scenario 1 from Invasion 1944. I was setting everything up and noticed that the Americans have off-board Artillery of 1x10 but there is not a 10 increment piece in the game piece palette.

In the conclusion for the scenario it states that the only fire support they had was 4.2 inch mortars. I looked at a 4.2 mortar piece and saw it had a fire strength of 10. I saw in one post on here regarding the log files a point about dropping the OBA counters on the target hex. I was wondering if it would be alright to use the 4.2 mortar piece as sort of a proxy OBA piece for that purpose.