Giving things a bit of thought along with the measurements you provided, you should find that the belt pitch is 5mm. This is based upon the fact that circumference of a circle is 2ᴨr (2*pi*radius) or ᴨD (pi*diameter). You measured the belt diameter as 50mm. Subtract the belt thickness (let's say it's 1mm) and you will calculate πD as 48mm * 3.1415... which equals 150.7964... This is the inner belt circumference (assuming a 1mm belt thickness). 150.7964... divided by 30 teeth (30T) gives a result of 5.02mm. This is the calculated tooth PITCH.
Additionally, you need to look closely at the shape of the teeth (tooth angle and height). From your pictures, your belt has a small recess between individual teeth. This suggests your belt is a polyurethane belt reinforced with steel/kevlar cords. I have attached a catalog page from SDP/SI (
https://www.sdp-si.com) to better explain what I am describing. Refer to Fig. 3. You will find that the belt described is a T belt.
Given all of the above, you should specifically be looking for a
T5 10mm wide 30T polyurethane belt.
Hope this helps.
DM