Arisaka sights

Does anyone know why some arisakas (38's) have a v notch back sight and some have a peep back sight

Im not an expert but I believe that it depends on what arsenal and time period the rifle was made in. I beieve late series rifles had cheaper sights due to production rushes and shortcuts. This was a progressive change that continued until the once respectable jap rifles were made to what we now refer to as "last ditch" standards. As I said I am not 100% positive and in no way do I claim to be an expert. Others will chime in...

Later type 38s introduced the peep rear sight which feature was also used on early to mid- production T99s (with the addition of the "wings" for leading aircraft on early T99s).

The late Type 38s did not suffer a reduction in quality as their production had ceased in Japan before the start of WWII.