There was a very old sticker on the safe bearing the name of the company who apparently installed it. The phone number was so old it did not have an area code. Fortunately the company still existed after multiple decades. I called them and asked if they could open it in a non-destructive way. One of their technicians came, looked at it and probed it for a couple hours, but determined he could not open it. And the combination had been changed from the manufacturer's default. He gave me the contact info for a reputed safe technician who could help.
Later I called this safe technician, but he was incredibly difficult to get a hold of. I had to leave multiple voicemails and send multiple emails. We chatted briefly one time and he said he would get back to me later to schedule an appointment. But he seemed half-retired and not interested in the job, as I never heard back, despite multiple contact attempts and my offer to pay handsomely. Eventually I became frustrated with his non-responsiveness and stopped caring about the safe.
Fast forward a few years later, I was going to sell the vacation home, but I really wanted to open the safe before selling. Curiosity had gotten to me. I searched online for another safe technician, and found a supposedly reliable guy. I arranged an appointment. He showed up a few days later. I asked him to open it any way he could, even if he had to destroy the safe. He started drilling, making multiple holes over the course of 2 hours. Eventually he came to me and said he ran out of drill bits as they all got worn out. He had to leave and promised he would be back.
It took one week for him to eventually come back early one morning with more drill bits. He spent another couple hours drilling. Then he put a camera scope in the holes and claimed he could see 3 of the 5 wheels spin while the other 2 were broken. He spent an entire day trying to manipulate the wheels. But after a whole day of work, he came to me with a defeated look and apologized saying he was sorry but he doesn't think he is able to open the safe.
I went back online to find yet another professional who could help. I learned that what I really needed to look for is a professional who is a member of SAVTA (Safe & Vault Technicians Association). So I found a SAVTA tech who on the phone told me a TL-15 safe in a residence is unusual as it is normally made for businesses like a jewelry store. Unfortunately he said his next availability would be about a month from now, and I was going to sell the house in the coming weeks.
Eventually I found another SAVTA tech who was available on a short notice. He and a colleague both arrived a morning, and it took them 3 hours to do more drilling and more manipulation to FINALLY open the safe.
Guess what was in it?
Nothing. It was empty! I closed the sale of the house literally 2 weeks later. I was still very relieved to have gone through this hassle to open it. The unsatisfied curiosity if it had not been open would have eaten me alive :) Also I decided in my next house I wanted a safe rated TL-15, as clearly they can withstand a lot.