Although I did find these references...
http://www.mortarsinminiature.com/USM4A1.htm General Vehicle Info:
Manufactured by the White Motor Co., the M4A1, a derivative of the M2 Half-Track Personnel Carrier, entered US Army service in May of 1943 with 600 vehicles being accepted into service. The M4A1 had improved suspension and had a higher combat weight over the M4 MMC. It was the first US Half-Track Mortar Carrier designed to allow for sustained main weapon firing from within the vehicle although, as delivered, the mortar still pointed towards the rear of the vehicle. The M4A1 provided for greater main weapon traversing, 600 mils vs. 130 mils. It was fitted with a front bumper winch which enabled it to self-extract itself. It served throughout World War II mostly (if not entirely) in the European Theatre of Operations. Had the numbers of M4 and M4A1 diminished sufficiently prior to the end of hostilities, it would have most likely been replaced by the M21 MMC. (When 10th Armored Division arrived in theatre in 1944, it arrived with several of the forward-firing M21 MMC.)
Assigned to Battalion Headquarters Companies in tank and armored infantry battalions, each company had a platoon of three mortar carriers and a command halftrack. The total vehicle compliment could range between 18 to 27 vehicles per armored division.
The M4A1 had a crew of six.
TM 9-710-55.jpg (47594 bytes)
http://www.mortarsinminiature.com/images/techmanuals/FM%2023-91.jpgLater variant info:
http://www.mortarsinminiature.com/USM21.htmAnd this one
http://www.wwiivehicles.com/usa/half-tracks/m4.asp
