Go here:
http://www.cyclepartsnation.com/oemparts/c/honda/parts
Look on the right side of the page. You'll see "Search By Part Number". Enter your number (no spaces). Click "Search". If it finds your part number, click on the next page "where used". You'll get a list of all the machines that use that part.
For reference, the number you listed showed the following results:
Assemblies where 51490-MG2-872 is used
1984 XL350R A - FRONT SHOCK ABSORBER
1984 XL350R AC - FRONT SHOCK ABSORBER
1984 XL600R A - FRONT FORK
1984 XL600R AC - FRONT FORK
1985 ATC250R A - FRONT FORK
1985 XL350R A - FRONT SHOCK ABSORBER
1985 XL350R AC - FRONT SHOCK ABSORBER
1985 XL600R A - FRONT FORK
1985 XL600R AC - FRONT FORK
1986 ATC250R A - FRONT FORK
1986 XL600R A - FRONT FORK
1986 XL600R AC - FRONT FORK
1987 XL600R A - FRONT FORK
1987 XL600R AC - FRONT FORK
Answer: Yes, these are for your 85 250R.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70