I have 3 numbers for this prom. Motorola part number QVLN4330A may be a factory programmed prom. Motorola part number 48M48 may be a blank prom but this number seems to be to short for the Motorola numbering system. The third number is an N82S123N. The prom may now be difficult to get. I can't guarantee the accuracy of any of the part numbers.
I seem to recall at one time when I was on the Internet coming across someone that can program the prom (other than an authorized Motorola service depot). The PROM is 256 bit, 32 X 8. The Motorola manual for the Morse Identifier Module is part number 68P02900A08-0. The manual has a program chart that provides enough information to allow someone with access to the appropriate prom programmer to program the call sign in manually. I used this information to program VE6PAX prom. My notes indicate that the first 2 address locations are left blank so the first letter (or number) of the call sign starts at address location 3. Reading an existing programmed prom may be useful to get a feel as to how it is programmed. A call sign letter or number may take up more than one address location.