NATCHEZ — Bill Rush Mosby and Sim Mosby aren’t a typical father-son pair.

Besides being a father and son duo of certified public accountants, the two have also been co-workers for the last two decades at Silas Simmons, LLP in Natchez.

Now, the elder Mosby will celebrate his 50th anniversary Tuesday, the same day his son will celebrate 20 years at Silas Simmons.

“At 22 years old I don’t imagine I was thinking much about celebrating a 50th anniversary anywhere,” Bill Mosby said.

He said much has changed in the time between his first day and the 50th anniversary, most notably, the technology used in the field.

On his first day in 1960, Bill Mosby said each desk was outfitted with a full-keypad adding machine and the office had two Friden calculators used for more complicated calculations.

“Now everything we do is on the computer,” he said. “If you gave a new CPA one of those machines we had, I doubt they would know what to do.”

Silas Simmons was established in Natchez in 1933.

Bill Mosby said the calculator technology was not the only thing that he has watched change during his tenure — photocopiers also joined the mix.

“We tried everything that came out, I think,” he said. “We even had this one machine that required you to hang up all the copies to dry.

“If we were using that technology today, we’d be hanging paper all over downtown.”

In 1990 after graduating from Ole Miss, Sim Mosby followed in his father footsteps by joining the staff at Silas Simmons, a move he has never regretted.

“As soon as I was offered the position here, I wanted to accept it,” Sim Mosby said. “They encouraged me to explore all of my options and offers, but I wanted to come back here.”

Bill Mosby said all of his children are certified public accountants and said while he never told them what the study in college he strongly encouraged them to pursue an accounting degree because of the diversity of uses.

“We are all math-minded I suppose,” he said. “Sim was the only one to come back to this environment, but the others are using the accounting background in other areas.”

While his 20-year tenure is only a fraction of his father’s 50-year stay, Sim Mosby said he has seen a fair share of changes in the industry including the evolution of computerized information databases.

“One of my first duties when I started here was to maintain the library,” he said. “At that time we had to subscribe to a large number of professional publications and when things would change I’d go through and make sure each volume was updated. Now all that information is online and all we have to do is make sure our subscriptions are renewed.”

Never worried about having a difficult working relationship with his father, Sim Mosby said he was eager to join the firm because he knew there was a wealth of information and experience from which he could learn.

He did say, however, he knew his father had created big shoes for him to fill.

“I was a little concerned when I started about following behind him,” Sim Mosby said. “He is a hard act to follow.

“He has helped me immensely, and now we have many clients that we both work with.”

In the last few years, there has been a switch in the dynamic between the Mosbys. Sim Mosby is a partner in the firm, and at 70, Bill Mosby vacated his title as a partner.

“I guess when he started I was his boss and now he is mine,” Bill Mosby said jokingly.

There are some perks that come with a 50-year tenure.

“He plays golf every Thursday afternoon,” Sim Mosby said of his father. “I guess after 50 years he has more than earned that freedom.”

Bill Mosby said when he first began maintaining a less demanding schedule he felt a small amount of guilt, but that has subsided.

“I even take a short day during tax season,” Bill Mosby said. “They can handle it.”