The N1 suffix you occasionally find on the end of Raltron part numbers is a callout for a part revision.
The standard part number CL2520-156.250-1.8-20-X-T-TR from Raltron has a Rise and Fall time of 1nS.
Part Number CL2520-156.250-1.8-20-X-T-TR-N1 has a Rise and Fall time of 0.5 nS
Applicable Part Number
CL2520-156.250-1.8-20-X-T-TR-N1
2151-CL2520-156.250-1.8-20-X-T-TR-N1TR-ND
2151-CL2520-156.250-1.8-20-X-T-TR-N1CT-ND
2151-CL2520-156.250-1.8-20-X-T-TR-N1DKR-ND