Hello. I'm new to the world of antique trucks. I've always wanted one - and I managed to purchase an early 1947 GMC EC152. She's beautiful. I got her running to discover I can't seem to get into all the gears - I'm pretty sure I can only get into 1st and 2nd. I was told it's a 4spd. The most concerning is I can't get her into reverse. She has an external/manual reverse lockout. When I lift the lever I can feel the shifter is both able to move further to the driver's side, and it appears to prevent engaging/entering forward gears.
According to the Novak website [https://www.novak-adapt.com/knowledge/transmissions/manual/sm420.html?srsltid=AfmBOopMeuxxgSA7UhXQ6AS99zZs4Tn8BcvHkhlbEPG8jxDI_AHx39GX] it appears to be an SM420 (side numbers are 590474 - top cover is 590511). Many websites indicate the SM420 only came with an external lockout (so maybe it's not a 420).
So my questions are...
1) How do I positively identify this tranny?
2) Does anyone know of a reason that I can't get into reverse (places to look or items known to fail)?
