Hi Stig. Is this one of the photos (Jan. 1943) on the St. Louis line; both aircraft are second in production but different Fiscal years.
As you say at some point in the last AT-9 and A-25 blocks they deleted the "dot"; the latest serials I've seen with them are 42-57061 (AT-9) and 41-18826 (A-25). In the other AT-9 block 41-5850 (near the end) had a standard serial.
https://www.docdroid.com/9tu76LJ/cur...os-pdf#page=13
No big deal but still interesting anomaly.
Nick