HOUSTON, Sept 29 (Reuters) - Exxon Mobil Corp (XOM.N) amended the contract proposal being made to locked-out workers at its Beaumont, Texas, refinery and lube oil plant to include some seniority protection, the company said on Wednesday. The offer marked the company"s first concession to the 650 workers at the 369,000-barrel-per-day refinery since a lockout began on May 1. The company previously insisted the union schedule a vote on its "last, best and final" offer made in January. Officials with United Steelworkers (USW) local 13-243 are reviewing the proposal, according to a person familiar with the matter. The USW has said the original offer would destroy the seniority system and create separate contracts for workers at the refinery and the adjacent lubricants plant. Exxon said it offered the revisions because the union has stressed it wants to maintain existing employee seniority provisions, according to a statement. The new proposal includes seniority protection for job transfers between the lubricants and refinery portions of the complex. It also includes layoff protection for certain jobs and warehouse operator promotions in lubricants, Exxon said. "The amended offer, which comes after months of good-faith bargaining meetings, provides further support to our represented employees’ concerns about security and seniority, while still achieving the company’s objectives of a safe, reliable operations, and a profitable site," spokesperson Julie King said in an email. If the new proposal is ratified before Nov. 1, the workers will receive a $500 bonus, Exxon said. In July, more than 75% of the locked-out workers rejected proposals to consider a vote on the contract. The refinery and lube plant has continued to produce Mobil 1 oil and motor fuels with managers, supervisors and some temporary operators hired since the lockout began. Exxon began the lockout to avoid a strike, the company has said.