I have a 1937 m20 3000 orig. miles totally original 1000 serial # came with check valve at base of pump. I helped work on very early 1937 300+ serial # had 2 bolt sump plate had no check valve at base of pump hollow tube. I also looked at a late 1937 engine 2000+ serial # had a slightly different pump base check valve tube approx. 1/8"to 1/4'different location from mid year pump if I remember correctly but that was 20 yrs. ago when I restored my 1937.