If reverse is left and then up towards dash (not down towards seat) and no lockout lever its definitely not the sliding gear 4 speed. Is the box you have synchronized? The sliding gear 4 speed boxes (also sometimes called crash boxes) are not synchronized. They also have straight-cut spur gears and have a noticeable "song" especially under a load in and in 1st, 2nd, and 3rd gears. Outward in appearance they are similar to the later SM-420 4 speed, but slightly smaller and shorter.
I posted this ride along a while back in the big bolt section, but it will give you an idea of what the sliding gear 4 speed sounds like. Turn up the volume for some gear whine!
Ride along. 
