Is posible to Change Display on Custom Transaction list/record Field from "Purchase Order 123/ABC/2026" to only "123/ABC/2026", netsuite included transaction type by default on dropdown list/record transaction field.

you can not, but fortunately you dont need that as the "%" wildcard can be used, so if you need to find the "123/ABC/2026" record just add % like "%123/ABC/2026" and the options will be filtered

Thanks bro.