Identifying my grandfather's Omega Geneve

Hi everyone. I'm hoping the community might be able to help me with a reference number for the Omega Geneve I inherited from my grandfather a few years back. After a 9-month wait for a Geneva-based Omega service, I just got it back last week. They did a beautiful job, especially on restoring the dial while retaining the custom Harley Davison logo. (For those who are curious, my grandfather worked at Harley and ordered these watches for himself and colleagues to commemorate the 75th anniversary in 1978.)

Unfortunately, the paperwork I received with the watch left the reference and serial number fields blank. There are no marks on the outside of the watch, including between the lugs. My understanding is that the marks are on the inside of the case back. The case is a unishell construction, so I don't have the ability or tools to open it myself. I have another watch currently being serviced by Nesbitt's right now, so I may just bring it up to them when I pick that one up. But if folks have an idea of the correct reference number, I'd be grateful. Thanks.