Serial number 43670 is a 1930 "small" pick guard OM-28 guitar. It was stamped on August 19, 1930 and cleared final inspection on October 15 that year. The small pick guard was used until replaced by the "large" pick guard in March 1931.
The Pages banjo tuners appear to be replacements since as far as is known Martin only used Grover #89 Deluxe banjo tuners on OM-28 guitars.
The top purfling is herringbone and the backstripe is the, usual for the period, zig-zag pattern.
Back and sides: Brazilian rosewood
Top: Red spruce (Adirondack)
At the time of writing (March 9, 2019) this guitar is avalable at Wexer Guitars: http://wexerguitars.com/